The new Opportune Critical looks good.
I agree, the "high jump bonus" is just strange. I suppose we could ditch it or try to come up with a simpler but thematically similar bonus. Maybe a bonus on the Opportune Critical?
A flat racial bonus would work for me. But I'm ok with some conditional, like putting leaving the Kick the same and changing Opportune Critical as so:
Opportune Critical (Ex): If an ovinaur rolls a critical hit with a melee attack, it can make an opportunity kick attack with a +4 [?] circumstance bonus to the attack roll. This additional attack counts as a normal attack of opportunity.
I think (a) no and therefore (b) no. As for (c), I always thought scoring a critical hit requires confirming it, but it doesn't hurt to specify. Maybe just change "rolls a critical hit" to "confirms a critical hit." And (d) but of course!
Those look good. I was going to suggest that the +X and +Y for the kick should be the same as for the gore attack, but then I got weirded out that these have such a low Str. Seems weird for a Medium centaur-ish critter. Anyway, what do you think about matching the kick to the gore?
We should definitely think about Weapon Finesse.
Anyway, shall we match the kick attack and damage modifiers to the gore modifiers?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.