according to this
http://divinity.gamepedia.com/Action_Pointsthere's such thing as "max" AP
Attribute cap is base 15; of course you can go over with gears.
high STR and DEX helps chance to hit (not damage) and boost the effectiveness of STR and DEX based skills.
that is, high STR and DEX won't let your melee and range characters do more damage.
high INT boosts all arcane skills and lower cooldown times
and yes, OoD boosts both spells and weapon damage.