What you want to do is go Pentence, Soul Power and get Divine Justice ASAP. One rank of Divine Justice gives you staying power, but it's not neccisary to rank it up until later in the game.

When it comes time that you cannot level up Pentence and Soul Power anymore, start putting points into Raise Dead Ward. This will probably start happening around level 7-9. Depending on if you needed to pick up shield for survival.

By level 15 with little to no damage gear. You will be hitting for around 400-500 a pop. But more importantly your damage is high throughout the game and you have the staying power with Divine Justice. Oak is a weird General, his army is more focused on making his normal attacks stronger and less about his army doing direct damage.

Honestly, the only Demigod that gives me troubles and makes me truly retreat from the battle field is UB.

levelling up to 5 is mostly with flag caps. once you're at level 5 you get Surge of Faith. combine Surge of Faith with rank 2 Divine Justice and you can blast through creep waves at turbo speed. your Surge will kill almost all of the creeps in the wave and instantly restore several hundred health and mana to yourself and everyone near you (through Divine Justice). the cooldown on it is quick, just 10 seconds, so you can do this repeatedly and progress through enemy creep waves at incredible speed. 

levelling from 5 to 10+ with Oak is very easy to do if you concentate on blasting creeps with Surge of Faith. once you get up to level 10ish you can start man-handling enemy DG's with a rank 3 or rank 4 Penitence and a Rank 2 or Rank 3 Shield. Shield and Penitence will give you huge dominance in DG dueling situations since you'll be able to interrupt and chase enemy DG's easily and be able to make your escape with almost zero risk of dying since you can shield on your way out. 

you mean parking the spirits in the back so they're out of harm's way and just sitting around for passive weapon damage boost?

i guess that works, but then you don't get the spirit attack damage so i'm not sure whether you end up on the plus side or minus side on the net. 

for those interested in increasing your micro manage skills for Generals with minions, the default hotkey to select just the general's own special minions (Spirits for Oak, Nightwalkers for Erebus, Shamblers for QoT, Yetis for Sedna) is the 'I' key. 

if you get good at microing your spirits you can probably weave them in and out of combat to prevent them from suiciding against towers or running into Circles of Fire, which is what they usually do if you're not watching them. 

I compiled a list of guides today for your viewing pleasure.  Right now Oak has 3 guides, all of which are assassin spec, as in not requiring idols.  I don't consider Raise Dead Ward to be a General spec because it directly increases Oak's damage, they don't really do anything else except take shots from a tower for you.

As for my list:

http://forums.demigodthegame.com/349958