It's not an “extra” o. It’s there because the English suffix is ‑ous not ‑us.
The OED notes the following as part of a very much longer etymological note:
Classical Latin stressed long ō /oː/ passed, via close /o/, into a
diphthong /ou/ in early Old French, which was variously written o or u,
less commonly ou; hence Latin adjectives in -ōsus, which either came down
in popular use, or were adopted at an early date, had in Old French forms
in -os, or -us (-ous), e.g. coveitos, coveitus, doleros, dolerus,
envios, envius, glorios, glorius, religios, religius.
In the 12th
cent. the vowel-sound changed in Central French (via 11th-cent. /eu/ and
later /øu/) to /ø/, written eu, so that the suffix had now the form -eus
(covoiteus, dolereus, envieus, glorieus, etc.); and this still later
was written in the masculine -eux (convoiteux, envieux, glorieux, with
feminine however in -euse), as still in modern French.
In Anglo-Norman the
forms were the same as in early Old French (coveitos, coveitus, envios,
envius, glorios, glorius), but in English the vowel was soon identified
with Old English and Middle English long ū, and like it written ou
(covetous, envious, glorious), the spelling ever since retained, though
the sound has passed through /uːs/, /us/, /ʊs/ to /ʌs/, /əs/ (there is some
evidence from early modern English orthoepists that where secondary stress
fell on the suffix, Middle English long ū was sometimes retained as the
diphthong /ʌu/, and that under low stress the final -s could be voiced to
/z/: see E. J. Dobson Eng. Pronunc. 1500–1700 (ed. 2, 1968) II. §281,
This -ous, having thus become the form of the suffix in all words
from Norman French, became the established type for all those of later
introduction, whether adaptations of French adjectives in -eus, -eux, or
Latin adjectives in -ōsus (but see -ose suffix¹), or new formations on the
analogy of these, from French, Latin, or other elements.
And later also:
This tendency to represent a Latin adjective by an English form in -ous may have been strengthened by the fact that the canonical form of the Latin adjective entered in dictionaries is the nominative singular masculine, and that this in the majority of adjectives ends in -us, the English pronunciation of which is the same as that of the English word in -ous, so that the latter to the cursory observer appears to be merely an English spelling of the Latin.
So all such words have the -ous; it’s not just analogous that merits is and so it somehow special.