I think I finally figured out how to make use of Oak. Instead of going with spirits and damage buff, instead go with a maxed out shield, his ranged slow and the AoE buff/damage.
I've always tried to play Oak in such a way that he was an effective pusher or demigod killer but in reality he is neither. However, when paired with an assassin, his debuff (that can be kept up 100% of the time) keeps the enemy slowed and taking +25% damage which is pretty huge paired with a shatter, smash or UB auto-attack.
On top of that if you are paired up with Regulus or UB (who depend on auto-attack for damage) the +20% movement speed and attack speed buff is pretty huge. Finally, you can use the shield to cover your own escape or keep an ally alive through a smash/shatter combo.
As far as playstyle, I try to actually stay in the back for the most part when up against Rook or Torch Bearer because if you are frozen or bouldered when the hammer comes down (or the shatter hits) your shield won't do anyone any good. Once those spells go off then I'll charge in and try to do a little damage.
End-game you will eventually get 10 spirits and +260 damage (+20 per spirit, +60 base) so you can dish out a little pain but by then the game is most likely decided.
Thoughts? Feedback? Flames?