The "oblique case" is everything that isn't in the nominative (cas sujet). If you look again at comes, you can see how this happened:
Latin: comes, comitis
N comes comites
G comitis comitum
D comiti comitibus
A comitem comites
Ab comite comitibus
In Vulgar Latin the cases started to blend together, because the final -m in the accusative singular and genitive plural was already heavily nasalized, so as to almost not be heard at all. (Hence, in Pompeii, graffiti exists that says, futui puella formosa instead of futui puellam formosam for "I fucked a pretty girl" ) Also, unstessed posttonic vowels often dropped out altogether, and the -s endings dropped (as in modern Italian).
As a result, the sound was:
N coms comte
G comti comtu
D comte comtiu
A comte comte
Ab comte comtiu
It's easy to see how the less educated might just start using "comte" for everything other than the nominative plural, and that's how declension systems die. Thus, by Old French, you see:
N li cons (or cuens) li comte (or conte)
O le comte (or conte) le comte (or conte)
As Old French developed, many of these irregular declensions then saw the -s ending on the oblique plural, and then the oblique version became the sole version:
le comte/le comtes (or les comtes, later)
ἡ δὲ κἀκ τριῶν τρυπημάτων ἐργαζομένη ἐνεκάλει τῇ φύσει, δυσφορουμένη, ὅτι δὴ μὴ καὶ τοὺς τιτθοὺς αὐτῇ εὐρύτερον ἢ νῦν εἰσι τρυπώη, ὅπως καὶ ἄλλην ἐνταῦθα μίξιν ἐπιτεχνᾶσθαι δυνατὴ εἴη. – Procopius
Ummaka qinnassa nīk!
*MySmiley*
