Oak is my favorite DG to play, although hes also the hardest. I won't say i'm a great oak player because i'm not. I get trounced often enough, but I've also done some serious trouncing.
As everyone knows, early game is Oaks biggest weakness. Midgame Oak can start causing havoc if he is allowed to pull too far ahead of you in equipment & levels. Endgame, with a full compliment of artifact gear and HoL I can camp your crystal and kill 2 to 3 DG's at once in front of your citadel. I don't often play games that get so far into it, but its fun when it happends.
The biggest trouble with playing Oak is that he is pretty slow, and pretty squishy. He is very gear dependent early on and desperately needs health and mana items to go toe to toe with anyone.
This means early game money is everything. If I can get 2 or 3 DG kills by level 5 or 6 and maybe a few assists - I'm in business.
The games where I've gotten absolutely raped have been when enemy DG's played smart and denied our team ANY kills whatsoever. Even if they don't kill us, the fact that Oak needs that infusion of cash generally puts him way behind the curve in terms of staying/killing power.
For most DG's don't be afraid to toe to toe with a solo Oak in the early game. Many times he will run and/or die (unless you're TB - Don't get close if you're TB. Range is your friend, and don't bother with your fire circle).
As an overall strategy just keep harrassing him and keeping him out of the lanes farming creeps as much as possible.
Another tip for generals - Don't bring tons of minions to help you post level 5. With Surge of Faith I can lay waste to your early minions and with Divine Justice they feed me health. The more powerful the minion, the more health they feed me.
Watch for penitence. Most oak players will use it early, and then again after the short cooldown. Wait to get hit with it before casting big dmg big cast time spells, or stay far enough outside his radius.
Pull oak away from the creep paths. Creeps are oak's best friend. They turn into his spirts and provide him health. Surge of Faith will kill weak creeps outright so a big creep wave will help him not hurt him.
Watch for shield. If i see rook doing his hammer slam I know to throw up my shield. If you see him turn shiny in the middle of a cast, just cancel the spell and wait 2 or 3 seconds. Shield has a high cooldown so once he's usd it, you know you're in good shape for another 30 seconds.
Late game, silencing artifacts and anti-heal auras are your best friend.
The nice thing about playing against oak is that since he is such a tricky DG to excel in you can rest assured that most Oak players are not top notch. Controlling his kills and his farming is the key to victory.
If at the end of the day you get beat by Oak, just rest assured you got outplayed/out teamworked - so chalk it up to a learning experience and try again.