Katana posted his build, so here is mine. This is my favorite Oak build. Lest you think it crap, it is playable and competitive. I've beaten Tier 1 players with it. I'll play no noob games with noobs on my team and it has carried me to wins. Here is my build, which took me a while to figure out:
1. Raise Dead Ward
2. Morale
3. Save
4. Ward 2/Morale
5. Shield 1
6. Morale
7. Ward 3
8. Morale
9. Save
10. Ward 4/Morale
11. Shield 2
13. Shield 3
14. Surge of Faith I
15. Surge II
Favor:
Ring of Divine Might is necessary without question. It's AoE allows easy farming of creeps, up to priests. The damage is fantastic as well. Horn of Battle got hit with a big nerf, so it's not that great anymore.
Items:
Priests
Gladiator gloves
Level I Minotaurs
Level I Seige Archers
Scaled Helm (We're going to sell it soon, so it doesn't count as a mana item.)
Forest Band
Boots of Speed
Ironwalkers
Wand of Speed
Gloves of Fell Darkur (Sell Scaled)
Cap locks
Combat mana potion
Side note: These item choices may not work if your opponents also stack speed or if your opponents have a snipe regulus. Adding in a few health items works well. In fact, you can stack all heath items and tank very well. This built is meant to take down towers in the fastest time possible. Adapt the items to your playstyle.
Playstyle
It's your job to destroy every single tower on the field. Early game, you will have to stick close to your flag. When you get a full complement of spirits, fling them at a tower along with your idol minions. Resummon and repeat. Stay out of direct contact of any Demigod, as they will kill you 1v1. By level 4-7, you should be able to use your minions to control a flag. Weaken a Demigod to about half health, and then confront him, especially if you have Shield I. You should also be fast enough to escape ganks when necessary.
Minion armor is a beast. Ironwalkers got reduced in price, and it is really good! Along with Forest band, you will be able to take out two or three towers with ONE minion wave at levels 7-10. If you are able to set up a Ward next to enemy towers, your creeps will get an additional 400 armor at level 7 and 500 armor at level 10. One creep wave will devastate multiple towers at this point. Morale provides a lot more armor, too. AA damage from towers will be pathetic.
You should prioritize your tower targets based on who your teamates are. Always take out the lane towers next to flags first. If you have some demigods that are good at ninjaing portals, then you should take out the side towers. If you have a TB, get him to freeze the towers in front of the citadel while your minions demolish them. After that, start flinging waves of minions at the citadel.
If you don't want to prioritize towers, that's fine. Stack HP gear with maybe 1 or 2 minion items. Your opponents will be burning sigils trying to escape your horde. It's hilarious and satisfying to watch them sigil from my measly lil' minions.
If you notice, this build only uses basic idols. The high tier idols are expensive and only improve 6 total minons. I would rather use that gold to invest in items that will make all of my minons superior. I can resummon idol minions frequently enough that I do not mind the fact that they die easily. Level 4 minotaurs, demolishers, and priests are sort of my "artifact" items.
Also, spam Ring of Divine Might. It does a fantastic job of farming until late midgame.
When not to use this build:
Don't use it against Ice TB or QoT. Ground spikes, Frost Nova, and Rain of Ice will wreak this build. Luckily, these are among the least used Demigods. Fire TB is maneagable. My minions are fast enough that they can usually skirt by Ring of Fire.
Don't use it if you're the only general (maybe). You aren't meant to tank or provide monk support early game. It's your job to knock shit down. It's much harder to use this build if your allies rely on monk support.
Where this build will own:
Any Demigod without significant AoE will get schooled by this build. That consists of everything besides the two Demigods above. The only caveat is making sure your teammates have monk support.
Another Oak may decide to copy you midgame. He likely won't have the Ring of Divine Might or Morale. He probably stacks health and mana items as well. These make him a pansy Oak. You will shred his towers faster than he shreds yours. You will be able to push him off flags with your minions and take his flags.
What I want you to do:
Start up a 1v1 on single player and use my items as shown. Set the starting level to level 10 and set starting gold on high. Buy as many items in my build as you can. Fling minions at towers, and see if they fall as fast as I say they do. Once you see the ownage, I think some of you may use this build. 
Happy razing!
