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^{_{Borrowing of words ending in -o from Italian, Portuguese, and Spanish, with retention of the final vowel, began in the early 16th cent. (e.g. Morisco adj., frizado n.). Previously, such forms had usually been assimilated to native phonological structure by the removal of the final vowel. Since unstressed final long vowels had not hitherto been part of English phonology, there was a tendency for the vowels o and a in this position to be confused; in spelling, final o frequently replaced a and other syllables (e.g. in camisado n. (Spanish camisada), calico n. (compare the place name Calicut), both from before 1550), and this tendency to substitute o continued into the 17th cent. and later (compare mango n.1 (Portuguese manga ), lingo n.1 (Portuguese lingua)); there are also English formations which appear to have been made in imitation of Romance loanwords (e.g. stingo n. and perhaps the nonce-word twango int.). By the late 17th cent., final -o seems to have become assimilated into English phonology and appears in new words of undetermined origin, e.g. bingo n.1, rhino n.1}}

^{_{The shortening of a word immediately after a medial o, and in particular where this occurs at the end of a prefix or combining form, first appears in the late 17th cent. and early 18th centuries, e.g. plenipo n., memo n., and hypo n.1 This probably established an association of the ending -o with casual or light-hearted use which it has retained ever since. Further examples are attested in the early 19th cent., e.g. (combining forms) Anglo n.1, mezzo n.1, typo n.; (other words) compo n.2, loco n.1 After 1851 this type of clipping becomes, and has remained, extremely common. Truncation after a written o not pronounced as such may be the explanation of tambo n.1 (mid 19th cent.), which, if not the first example of the use of the suffix -o, seems to anticipate its later pattern of use with truncated word-forms.}}

^{_{The attachment of ho int.1, O int., and oh int. to other words to form conventional cries and refrains is attested from late Middle English, e.g. in heave ho int. and n., hey-ho int.; alive ho! is attested from the early 18th cent. or earlier (see (all) alive, (alive), oh! at alive adj. Phrases 2); and expressions such as righto int. and n., billy-o n., and cheerio int. are attested from the later 19th and early 20th cent. A number of words occur in the second half of the 19th cent. which appear to have their origin in the attachment of one of these interjections to a noun or adjective to form a refrain-like or vocative expression (such as a form of address or a public announcement) which was subsequently used as an ordinary noun or adjective (compare spell oh! (or ho!) at spell n.3 3c). Some of these are normally or frequently written as hyphenated compounds of -ho , -O or -oh, e.g. smoke-ho n., dead-oh adj. (late 19th cent.), bottle-o(h n., rabbit-o n. and int. (early 20th cent.), daddy-o n. (mid 20th cent.), while others (see below) are treated in spelling as having the suffix -o; their classification as cases or non-cases of the present suffix is rather arbitrary. The earliest example which shows a clear transition from one use to the other is milko int. and n.; other examples of the use of the suffix in similar formations are kiddo n. and Relievo n.2 (a game named after the cry used in it) (late 19th cent.), whizzo int. and adj., and socko int., adj., and n. (first half of 20th cent.).}}

^{_{From the early 20th cent. the addition of the suffix to complete words to form nouns, adjectives, and interjections of all kinds becomes very common, e.g. wino n.1, whacko int., and cheapo adj. The earliest example of the addition of the suffix to a truncated word is probably beano n. (second half of 19th cent.), followed by Salvo n.3, an Australian formation; another (uncertain) example from the same time and place is robbo n. Since the beginning of the 20th cent. formations of this kind have become numerous, e.g. ammo n., arvo n. (one of several later Australian formations), and wacko adj.}}

^{_{The suffix is not infrequently used to create product names, among the earliest being blanco n. and Oxo n. in the late 19th cent.}}

Borrowing of words ending in -o from Italian, Portuguese, and Spanish, with retention of the final vowel, began in the early 16th cent. (e.g. Morisco adj., frizado n.). Previously, such forms had usually been assimilated to native phonological structure by the removal of the final vowel. Since unstressed final long vowels had not hitherto been part of English phonology, there was a tendency for the vowels o and a in this position to be confused; in spelling, final o frequently replaced a and other syllables (e.g. in camisado n. (Spanish camisada), calico n. (compare the place name Calicut), both from before 1550), and this tendency to substitute o continued into the 17th cent. and later (compare mango n.1 (Portuguese manga ), lingo n.1 (Portuguese lingua)); there are also English formations which appear to have been made in imitation of Romance loanwords (e.g. stingo n. and perhaps the nonce-word twango int.). By the late 17th cent., final -o seems to have become assimilated into English phonology and appears in new words of undetermined origin, e.g. bingo n.1, rhino n.1

The shortening of a word immediately after a medial o, and in particular where this occurs at the end of a prefix or combining form, first appears in the late 17th cent. and early 18th centuries, e.g. plenipo n., memo n., and hypo n.1 This probably established an association of the ending -o with casual or light-hearted use which it has retained ever since. Further examples are attested in the early 19th cent., e.g. (combining forms) Anglo n.1, mezzo n.1, typo n.; (other words) compo n.2, loco n.1 After 1851 this type of clipping becomes, and has remained, extremely common. Truncation after a written o not pronounced as such may be the explanation of tambo n.1 (mid 19th cent.), which, if not the first example of the use of the suffix -o, seems to anticipate its later pattern of use with truncated word-forms.

The attachment of ho int.1, O int., and oh int. to other words to form conventional cries and refrains is attested from late Middle English, e.g. in heave ho int. and n., hey-ho int.; alive ho! is attested from the early 18th cent. or earlier (see (all) alive, (alive), oh! at alive adj. Phrases 2); and expressions such as righto int. and n., billy-o n., and cheerio int. are attested from the later 19th and early 20th cent. A number of words occur in the second half of the 19th cent. which appear to have their origin in the attachment of one of these interjections to a noun or adjective to form a refrain-like or vocative expression (such as a form of address or a public announcement) which was subsequently used as an ordinary noun or adjective (compare spell oh! (or ho!) at spell n.3 3c). Some of these are normally or frequently written as hyphenated compounds of -ho , -O or -oh, e.g. smoke-ho n., dead-oh adj. (late 19th cent.), bottle-o(h n., rabbit-o n. and int. (early 20th cent.), daddy-o n. (mid 20th cent.), while others (see below) are treated in spelling as having the suffix -o; their classification as cases or non-cases of the present suffix is rather arbitrary. The earliest example which shows a clear transition from one use to the other is milko int. and n.; other examples of the use of the suffix in similar formations are kiddo n. and Relievo n.2 (a game named after the cry used in it) (late 19th cent.), whizzo int. and adj., and socko int., adj., and n. (first half of 20th cent.).

From the early 20th cent. the addition of the suffix to complete words to form nouns, adjectives, and interjections of all kinds becomes very common, e.g. wino n.1, whacko int., and cheapo adj. The earliest example of the addition of the suffix to a truncated word is probably beano n. (second half of 19th cent.), followed by Salvo n.3, an Australian formation; another (uncertain) example from the same time and place is robbo n. Since the beginning of the 20th cent. formations of this kind have become numerous, e.g. ammo n., arvo n. (one of several later Australian formations), and wacko adj.

The suffix is not infrequently used to create product names, among the earliest being blanco n. and Oxo n. in the late 19th cent.

I should have checked the OED3; it has significantly expanded its treatement. They seem much more certain now, citing three distinct origins, then following that up with significant discussion in their finest print, and then their four distinct senses. Skip the fine print the first time you read this.

^{_{The suffix is not infrequently used to create product names, among the earliest being blanco n. and Oxo n. in the late 19th cent.}}

Forming slang and colloquial nouns, adjectives, and interjections. ^{_{The use of the suffix is widespread in English-speaking countries and is especially associated with Australia.}}

1. Forming interjections, as whacko int., whammo int.

2. Forming familiar, informal equivalents of nouns and adjectives, as (from truncated word-forms) aggro n. and adj., combo n., metho n.1; (from complete words) bucko n., kiddo n.; cheapo adj., neato adj.

3. Forming personal nouns from non-personal nouns, as milko n., wino n.1

4. Forming nouns from adjectives, as pinko n., weirdo n.

Edit

I should have checked the OED3; it has significantly expanded its treatment.

They seem much more certain now, citing three distinct origins, then following that up with significant discussion in their finest print, and then their four distinct senses. Skip the fine print the first time you read this.

(Here is where the fine print occurs.)

Forming slang and colloquial nouns, adjectives, and interjections. ^{_{The use of the suffix is widespread in English-speaking countries and is especially associated with Australia.}}

1. Forming interjections, as whacko int., whammo int.

2. Forming familiar, informal equivalents of nouns and adjectives, as (from truncated word-forms) aggro n. and adj., combo n., metho n.1; (from complete words) bucko n., kiddo n.; cheapo adj., neato adj.

3. Forming personal nouns from non-personal nouns, as milko n., wino n.1

4. Forming nouns from adjectives, as pinko n., weirdo n.

Fine Print

^{_{The suffix is not infrequently used to create product names, among the earliest being blanco n. and Oxo n. in the late 19th cent.}}

Examples

I don’t think anybody quite knows. The OED gives four distinct types of these formations. They are wondering whether it has something to do with the O interjection with some reinforcement from similar words. They also note that it is nowhere more common than in Australia.

The four different types they talk about may or may not come from the same linguistic impulse, either. Further research is called for.

An excerpt from the OED2 is given below.

-o, suffix². Perh. connected with O int.³ and reinforced by the final syllable of abbrev. forms such as compo, hippo, photo, etc. The use of the suffix is widespread in English-speaking countries but nowhere more so than in Australia (e.g. afto, arvo, Commo, etc.).

• a. Forming colloq. or slang equivalents added as a final syllable to (a).shortened forms of sbs., as aggro, ammo, beano, combo, compo², metho¹, etc.; (b).sbs., as boyo, bucko, kiddo, etc.; (c).adjs., as cheapo, deado.

• 1967 J. Burke Till Death us do Part ii. 31 ― You can buy that cheapo, cos no one wants it.

• 1969 It 13-25 June 16/2 ― Hustle the bread from whatever source you can··. If all the above sounds like too much aggro don’t··go and··run your benefit event in conjunction with an existing club.
• 1969 It 10-23 Oct. 10/1 ― At the moment kids are split up into different subcultural groups which have been driven by the system into a permanent state of aggro with each other.
• 1969 Daily Mail 8 Nov. 8/3 ― How do we get past him, man? Like he might start some agro.
• 1970 Observer 11 Jan. 28/2 ― Hippies and aggro-boys may look collectively and individually startling, pretty or repulsive according to tribal loyalty.
• 1973 A. Hunter Gently French iii. 29, ― I gets hold of the bastard and tries to pull him up. Then I sees he’s bloody deado.
• 1973 M. Amis Rachel Papers 65 ― It wasn’t day-to-day aggro, nor the drooped, guilty, somehow sexless disgruntlement I had seen overtake many relationships.
• 1977 E. Crispin Glimpses of Moon xi. 215 ― It was possible to judge··that his aggro was strictly verbal.

• b. Forming personal (chiefly occupational) sbs. from non-personal sbs., as bottle-o(h, milko, wino, etc.

• c. Forming sbs. from adjs., as pinko, weirdo, etc.

• d. As a meaningless ending in other words, as billy-o, good-o, cheerio, right(y)o, etc.

I should have checked the OED3; it has significantly expanded its treatement. They seem much more certain now, citing three distinct origins, then following that up with significant discussion in their finest print, and then their four distinct senses. Skip the fine print the first time you read this.

Etymology: < -o, of various origins:

• (i) as the final syllable of words of chiefly Romance origin;

• (ii) as the vowel that became final after the shortening of a word by dropping the syllables following a medial o, especially in compounds truncated after a prefix or combining form ending in -o ; and
• (iii) < ho int.1, O int., and oh int., occurring as a second element in various exclamatory phrases. The suffix is attached both to full words and to truncated forms of words and phrases.

^{_{Borrowing of words ending in -o from Italian, Portuguese, and Spanish, with retention of the final vowel, began in the early 16th cent. (e.g. Morisco adj., frizado n.). Previously, such forms had usually been assimilated to native phonological structure by the removal of the final vowel. Since unstressed final long vowels had not hitherto been part of English phonology, there was a tendency for the vowels o and a in this position to be confused; in spelling, final o frequently replaced a and other syllables (e.g. in camisado n. (Spanish camisada), calico n. (compare the place name Calicut), both from before 1550), and this tendency to substitute o continued into the 17th cent. and later (compare mango n.1 (Portuguese manga ), lingo n.1 (Portuguese lingua)); there are also English formations which appear to have been made in imitation of Romance loanwords (e.g. stingo n. and perhaps the nonce-word twango int.). By the late 17th cent., final -o seems to have become assimilated into English phonology and appears in new words of undetermined origin, e.g. bingo n.1, rhino n.1}}

^{_{The shortening of a word immediately after a medial o, and in particular where this occurs at the end of a prefix or combining form, first appears in the late 17th cent. and early 18th centuries, e.g. plenipo n., memo n., and hypo n.1 This probably established an association of the ending -o with casual or light-hearted use which it has retained ever since. Further examples are attested in the early 19th cent., e.g. (combining forms) Anglo n.1, mezzo n.1, typo n.; (other words) compo n.2, loco n.1 After 1851 this type of clipping becomes, and has remained, extremely common. Truncation after a written o not pronounced as such may be the explanation of tambo n.1 (mid 19th cent.), which, if not the first example of the use of the suffix -o, seems to anticipate its later pattern of use with truncated word-forms.}}

^{_{The attachment of ho int.1, O int., and oh int. to other words to form conventional cries and refrains is attested from late Middle English, e.g. in heave ho int. and n., hey-ho int.; alive ho! is attested from the early 18th cent. or earlier (see (all) alive, (alive), oh! at alive adj. Phrases 2); and expressions such as righto int. and n., billy-o n., and cheerio int. are attested from the later 19th and early 20th cent. A number of words occur in the second half of the 19th cent. which appear to have their origin in the attachment of one of these interjections to a noun or adjective to form a refrain-like or vocative expression (such as a form of address or a public announcement) which was subsequently used as an ordinary noun or adjective (compare spell oh! (or ho!) at spell n.3 3c). Some of these are normally or frequently written as hyphenated compounds of -ho , -O or -oh, e.g. smoke-ho n., dead-oh adj. (late 19th cent.), bottle-o(h n., rabbit-o n. and int. (early 20th cent.), daddy-o n. (mid 20th cent.), while others (see below) are treated in spelling as having the suffix -o; their classification as cases or non-cases of the present suffix is rather arbitrary. The earliest example which shows a clear transition from one use to the other is milko int. and n.; other examples of the use of the suffix in similar formations are kiddo n. and Relievo n.2 (a game named after the cry used in it) (late 19th cent.), whizzo int. and adj., and socko int., adj., and n. (first half of 20th cent.).}}

^{_{From the early 20th cent. the addition of the suffix to complete words to form nouns, adjectives, and interjections of all kinds becomes very common, e.g. wino n.1, whacko int., and cheapo adj. The earliest example of the addition of the suffix to a truncated word is probably beano n. (second half of 19th cent.), followed by Salvo n.3, an Australian formation; another (uncertain) example from the same time and place is robbo n. Since the beginning of the 20th cent. formations of this kind have become numerous, e.g. ammo n., arvo n. (one of several later Australian formations), and wacko adj.}}

^{_{The suffix is not infrequently used to create product names, among the earliest being blanco n. and Oxo n. in the late 19th cent.}}

Forming slang and colloquial nouns, adjectives, and interjections. ^{_{The use of the suffix is widespread in English-speaking countries and is especially associated with Australia.}}

1. Forming interjections, as whacko int., whammo int.

2. Forming familiar, informal equivalents of nouns and adjectives, as (from truncated word-forms) aggro n. and adj., combo n., metho n.1; (from complete words) bucko n., kiddo n.; cheapo adj., neato adj.

3. Forming personal nouns from non-personal nouns, as milko n., wino n.1

4. Forming nouns from adjectives, as pinko n., weirdo n.