I've been meaning to write a guide again.. but.. a couple things to keep in mind.. everything about how I play is situational and I change things depending on the situation.  While the build is mostly set until after 15, the item order and general strategy change each game.

1-3. Shield / Pen / DJ - Get w/e order you personally want.  If you expect to fight in the middle, shield is a great lvl 1 skill.  If you're the double on the fort flag, shield is a great lvl 1.  But, fighting 1v1 on mana, pen first can be more helpful.

4. Pen II

5. Surge I

6. Shield II

7. Pen III.  Yes, I ALWAYS get Pen III first.. don't get into a situation that you need shield III to get out of until you hit 8.. you can wait that long, but the additional damage from Pen III provides more benefit if you don't play stupid.

8. Shield III

9. Save

10. Surge II / Pen IV

11. Raise Dead Ward I

12. Ward II

13. Ward III

14. Ward IV

15. Surge III

16. Soul Power I

17-20 - Morale, DJ, or Shield.  Shield max can be helpful if enemy has debuffs you want to keep off, morale is great for more damage, DJ is nice to spam more mana but I almost never get more DJ due to lack of mana issues this late.

Favor:  BotF, or Cloak of the Night

I rarely pick cloak, but it's a popular choice with others (which I usually see is their downfall).  It's nice to counter occ / ere jump, but I'll usually just get botf and use j-treads to catch them.

Lane: Ideally you can get mana lane at the beginning.  However, this is very team dependent.  You are one of the best choices for mana tho (I might write up something on lane assignments in another thread).

Starting Item: Monks*.  Save for a tp only if you think the other team is likely to push your teammates down behind a tower.

*If 2 generals, make sure the other general knows to get monks, and get scaled helm / banded armor.  You will get clerics (1500g) as soon as you go back shopping (preferably when you have 2300g to get clerics, a tp, and fs1).

Next shopping trip is very dependent on how the game has gone thus far.  Is your hp fine to push your lane?  Then get mana.  If not, get unbreakables.  Ideallly you will have ~ 3250g when you first go back and can get plenor battlecrown, unbreakable boots, and a tp.  However, if the other team is overly aggressive, it is better to go back around 2350g to get 1 of those items, fs1, and a tp.  If you have any money in between any of this, get w/e you can and get scaled helm as a little item.

So, General 1st shopping trip (assuming monks on first trip):

-1 1500+ Helm and / or Unbreakable boots

-1 tp

After this point, you need to get lvl 7 immediately if you haven't already gotten it.  Pen III and some mana to use it is where you finally start doing something.  At this point, if lane assignments are still working out, then keep doing what you're doing until lvl 10, where the TRUE power of Oak begins and the game changes.  If you think you need a sigil to fight @ lvl 7, get one.  If you just recently came from the shop, a great way to go back is to spam all of your mana fighting someone out or pushing down a tower as low as you can get it without dying to a gank.  You'll have accomplished tower damage or severely hurting the other person and can safely go back to base.  Always have a tp in stock to save / gank.  If you're playing aggressively, get 2 tp's so that you can tp in to a flag, push someone all the way back behind their towers, and be able to tp out again.  Beyond lvl 10 always carry 2 tp's and a sigil.

Next Items:

Again, your shopping will be sporadic now.  Have these items lined up (again, the order is dependent on whether you need more hp or more mana)

-Unbreakable boots or 1 helm asap if you didn't get both on first trip

-2nd 1500+ Helm

-Nimoth Chest

-Hauberk of Life

-Bishops / high priests (only if you need them over monks.  I typically only get if my monks die often to aoe like a tb / occ / rook / reg).  

-Demolishers (I don't often get these.  But I probably should get them more.. they are incredibly useful, both to tear towers down and they do a decent amount of damage on your target when you have pen / surge going)

-Little items to fill slots:  Banded armor / Scaled Helm.  Rarely I will keep 3 helms to spam abilities a LOT, but eventually you should sell.  However, not worth selling banded to pick up any other item on the above list.

Beyond this point, you should be all over the map.  If left alone, push any tower in sight and be prepared to shield / port out.  Don't be afraid to continue pushing down the tower as low as you can safely get it even if 1 person tp's in.  As long as you have enough hp to survive w/e stun they have so you still have time to shield.  Keep your spirits with you and just use them for additional damage on your target or to absorb tower damage.  Don't send them off on their own to attack someone or a tower, they are very weak.

Portal Game Items:

-J-treads (6750g):  I tend to get these fairly often, even sacraficing creep upgrades to get.  If you are able to successfully get a portal, these allow you to port into the flag, shield, lock, and run away safely.  W/out them, you have to run there, shield / lock / port.  This leaves you little else to do on the map except keep the portal, and you might not make it to the portal if someone is in the lane to slow you down on your way there.  They are also AMAZING for chasing down erebus / occulus after they jump / swarm.  Yes, you can catch a double jumping occ and a batswarm II erebus.

-Hungarling's Crown (5500g):  If I find myself constantly running oom yet still needing high health, I'll get this and sell vlemish helm.  This is a GREAT team item as well.

-Groffling Warplate (5200g):  Helps to push towers down and for walking through tower farms.  If I don't need, and don't think I'll need the speed from J-treads, and my mana is fine, I'll get this instead.  Allows for great staying power.

Obviously at this point you should have as many tp's and locks as possible, and 1 sigil.

Get cit upgrades as necessary.. I usually like to put most of these off to my teammates as I feel confident in my ability to fully utilize the power portal game items give me.  However, feel free to take a decent kit and then save for creep upgrades.  I also do whatever I can to not buy cur1 unless there is someone else on my team who I think can be stronger than myself (refer to Wrong Person Buying Currency section here).