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Ending in -t, -te, -s or -se is mostly necessary for a verb to form an agent noun in -or (with exceptions, as discussed above), but it certainly isn't sufficient. Sven Yargs left a comment mentioning "disrupter" as an example of an agent noun that is commonly spelled with -ter (although the spelling disruptor also exists). User and passer are always spelled with -ser.

Ending in -t, -te, -s or -se is mostly necessary for a verb to form an agent noun in -or (with exceptions, as discussed above), but it certainly isn't sufficient.

• Sven Yargs left a comment mentioning "disrupter" as an example of an agent noun that is commonly spelled with -ter (although the spelling disruptor also exists). Adopter, corrupter, tempter, respecter are other difficult-to-predict common spellings in -ter.

• User and passer are always spelled with -ser.

The final consonant of the verb is relevant, indirectly, because of the etymology of the ending -or. However, there is no simple rule that always gives the right ending, because there are some complications due to both etymology and analogy.

Latin perfect passive participle stems characteristically end in t or s. So in practice, verbs that form agent nouns in -or almost always end in -t(e) or -s(e), and form agent nouns in -tor or -sor specifically. In the context of Latin, the suffix is often given as -tor rather than as -or.

The final consonant of the verb is relevant, indirectly: because of the etymology of the ending -or, it is usually found after t or s. However, there is no simple rule for -or vs. -er that always gives the right ending, because there are some complications due to both etymology and analogy.

Latin perfect passive participle stems characteristically end in t or s. So in practice, verbs that form agent nouns in -or almost always end in -t(e) or -s(e), and form agent nouns in -tor or -sor specifically. In the context of Latin, the suffix is often given as -tor rather than as -or. (There is an unrelated Latin suffix -or that forms abstract nouns from verbs, such as error, rigor, terror).

There are agent nouns in English that end in -or preceded by a letter other than t or s. The main origin of this type of -or agent noun is French. In Old French, the t in the Latin ending -atorem (as well as some other similar endings) was lost by regular sound change, and the syllables merged, causing it to become -or/-our/-ur (Modern French -eur). This was taken into Middle English as an agent-noun suffix -our.

In Modern English, this ending came to be unstressed and its vowel became reduced, making it sound identical in English pronunciation to the -or ending found in Latin words, and its spelling has therefore been simplified in most cases to -or.

• A large proportion of -or agent nouns only occur in legal English, where -or is used more than in regular English (often as a counterpart to -ee). For example, "deliveror" and "settlor" are basically only found in legal contexts; in other contexts, the spellings "deliverer" and "settler" are used.

• Some -or agent nouns that are not restricted to legal English are conquer-or, purvey-or, survey-or, counsel-(l)-or, and vend-or (the last has a less common but accepted variant vend-er).

• sail-or is an example of the French suffix being used on an English base, the verb sail.

There are other -or words that don't end in -tor or -sor, but most of the rest aren't built directly on an English verb as a base. For example, donor, emperor, tailor, juror don't refer to people who "done", "emper", "tail" or "jure".

There seems to be sporadic use of the form -or instead of -er simply as a means of forming words with a specialized meaning, sometimes as part of technical terminology or jargon.

There are agent nouns in English that end in -or preceded by a letter other than t or s. The main origin of this type of -or agent noun is French (specifically, the Anglo-Norman variety that was spoken in England after the Norman conquest). In Old French, the t in the Latin ending -atorem (as well as some other similar endings) was lost by regular sound change, the a turned into a schwa (first spelled e and then eventually lost), and as a result the whole thing ended up as -or/-our/-ur (Modern French -eur). This was taken into Middle English as an agent-noun suffix -our. A simplified example: Latin gubernator (accusative gubernatorem) became Anglo-Norman French gouvernour, Middle English governour, Modern English governor.

In fact, there was extensive spelling variation and some confusion with other suffixes of similar meaning already in French, so the Oxford English Dictionary lists not only gouvernour but also governor, governour, governur, guvernur, governer, guverner as possible spellings of the Anglo-Norman word, and couernour, couernoure, gouernowre, gouernur, gounour, gouvernour, guuernere, guuernour, governore, gouernere, governowre, gouerner, gouernor, gouernour, gouernoure, governoure, governour, governer as potential spellings in Middle English.

The ending -our was pronounced /uːr/ in Middle English and was at least sometimes stressed (Chaucer rhymes governour with flour 'flower'¹) but in Modern English, this ending came to be unstressed and its vowel became reduced, making it sound identical in English pronunciation to the -or ending found in Latin words. Along with the preexisting variation and the analogical influence of Latin -or, this made it easy for the spelling -our to be simplified in most cases to -or.

• A large proportion of -or agent nouns only occur in legal English, where -or is used more than in regular English (often as a counterpart to -ee). For example, "deliveror" and "settlor" are basically only found in legal contexts; in other contexts, the spellings "deliverer" and "settler" are used.

• Some -or agent nouns that are not restricted to legal English are conquer-or, purvey-or, survey-or, counsel-(l)-or, and vend-or (the last has a less common but accepted variant vend-er). These all originate from (Anglo-Norman) French nouns, but can be reinterpreted in modern English as suffixed derivatives of the English verbs conquer, purvey, survey, counsel, vend.

• sail-or is an example of the French suffix being used on an English base, the verb sail.

There are other -or words that don't end in -tor or -sor, but most of the rest cannot be interpreted as being built directly on an English verb as a base. For example, donor, emperor, tailor, juror don't refer to people who "done", "emper", "tail" or "jure".

There seems to be sporadic use in English of the spelling -or instead of -er simply as a way of forming words with a specialized meaning, sometimes as part of technical terminology or jargon.

¹"Appollo, god and governour / Of every plaunte, herbe, tree and flour"—"The Franklin's Tale"