Oak is pretty weak early game, and he is one of the three demigods who BREATHES on levels: DA,TB,Oak. In order to compete with any tier 1 demigod they basically need to either be at a 1 or 2 level advantage or they need to be the same level, but a high level (say, 12 or so).
Assuming you're an Assassin Oak... the important thing for Oak is basically not to shop, if you at all can, until you have at least 4400 gold and level 8, though 3250, level 7 is also a good number. This means you aren't getting regen or currencfy which is something to really take into account, esp in your typical pug games. Until then, go mana, farm capping the mana and XP flags and kill all the creeps. Just power level the hell up.
Then, get Vlemish+Plenor, Banded+Scailmale, and some ports/sigils/locks whatever and rape face. For beast you need to really watch the creep waves and make sure you surge 'em for HP and a little extra push.
Also, remember the attack speed debuff of Ooze does'nt work when you're shielded. I know that blowing your shield halfway through combat is always dangerous, but UB's always have a sigil ( you should too of course 
). Regardless, they pop a sigil, pop your shield. Shield is a third of the duration of a siggy for one thing and secondly if he is sigiling anyway he is probably losing and the shield should be just enough to push him away.
You can always shield away spit, etc. Just don't OPEN with a shield, that's a bad, bad idea.
Anyway, just power-level to win
