Pros- High close ranged AOE damage, tanky, can lock as osa or with gobballs, little bit of mp reduction, constant pulls. Earth osa can also utilize tofu form pretty well if needed. White gobball is probably the best summon to give high-energy shot to.
Cons- You have minus dodge when in gobball form, so you'll be locked by literally everything. You're also pretty useless on high ranged maps. Also despite gobball osa being the tanky form, it just doesn't seem tanky enough compared to other classes.
Pros- Decent heals, constant unbewitchment, dragons have long range damage, intell osa has the highest range compared to the other 2 builds, dragon form gives osa summons 2 ap, which allows for tofu and gobball to hit twice rather than once. (this extra hit really adds up). The best form for supporting with heals, buffs and summons.
Cons- Intell osa deals little damage while in dragon form (you lose 20% damage) so you have to rely on summons for damage. This isnt really a con since summon spamming in solo quest fights and kolo is what an osa is best at, but for pvm having a bunch of uncontrollable animals isnt the best thing (gobballs and tofus).
Black dragon is extremely fragile and is probably the worst osa summon (but still good)
Overall though, for kolo the best would be intell osa because the heals are really nice for your team. Also most kolo maps are really high ranged so strength osa might struggle on those maps. But for quests earth osa will be able to make quick work because of the AOE.