We should definitely think about Weapon Finesse.
Yup! We can leave that until we do the feats, or should that be feat?
Anyway, shall we match the kick attack and damage modifiers to the gore modifiers?
Hold on a second. That gore attack is a holdover from the Caprine. Ovinaurs don't have gore attacks. Indeed, it's unclear whether they have horns - they're basically a centaur arrangement of a halfling from the waist up on a goat body.
I'm strongly tempted to putting a regular hooves attack in their Attack/Full Attack line, since kicking seems to be their thing.
In which case we'd have something like:
Attack: Weapon +2 melee (??) or hoof +2 melee (1d4–1)
Full Attack: Weapon +2 melee (??); or 2 hooves +2 melee (1d4–1)
Opportunity Kick (Ex): An ovinaur can strike with both its front hooves or both its rear hooves as an attack of opportunity. It resolves this action with a single attack roll, dealing 2d4 damage plus 1½ times the ovinaur’s Strength bonus. A typical ovinaur's opportunity kick is attack bonus +2 melee, damage 2d4–1.
Opportune Critical (Ex): If an ovinaur confirms a critical hit with a melee attack, it can make an opportunity kick attack with a +4 circumstance bonus to attack against the target of that critical hit. This additional attack counts as a normal attack of opportunity.