there are 2 primary builds of Oak, you'll find variation but here they are, in short

Build 1: Raise Dead Ward focused, emphasis on minions and lane pushing

1 - Raise Dead 1

2 - Soul Power 1

3- Divine Justice 1

4 - Raise Dead 2

5 - Surge of Faith 1

6 - Shield 1

7 - Raise Dead 3

8 - Penitence 1

9 - Last Stand 1

10 - Raise Dead 4

11- Surge of Faith 2

12+ - build is basically complete, flesh out with whatever you like, common to max Morale to increase minion damage

Build 2: Penitence Focused, emphasis on killing enemy DG's with direct attack

1 - Penitence 1

2 - Divine Justice 1

3 - Shield 1

4 - Penitence 2

5 - Surge of Faith 1

6 - Shield 2

7 - Penitence 3

8 - Shield 3

9 - Last Stand 1

10 - Penitence 4

11 - Surge of Faith 2

12 - Shield 4

13+ - build is basically complete, common to max out Divine Justice and get Rally for decisive endgame DG duels

for Build 1 your equipment choices will generally be more focused on minions. you'll get minion boosting armor like Plate of the Crusader. minion boosting damage gear like Gloves of Fell Darkur. maybe you got a minion boosting Favor Item like Ring of Divine Might. you'll probably eventually try to buy all 3 rank 4 Idols. your endgame outputs massive sustained damage with a minion army put you're not very good at ganking enemy DGs since its relatively easy to run from you. 

for Build 2 your equipment choices are generally tanking gear and mana gear. you stay in combat for a long time so you need to take hits and you spam Penitence so you need alot of mana. you've also got your Shield trick so Heart of Life is top priority. you're probably taking Blood of the Fallen as your favor item. probably getting Nimoth Chest Guard, Vlemish Faceplate, and Heart of Life as your 3 highest priority equipment picks. you probably don't care much about your minions for damage but are very likely to get the rank 2 or rank 3 priest Idol because the healing is very useful. 

i've tried it, it doesn't work.

you can definitely squeeze the skill points in, you end up with this mix at level 12: Penitence 4, Raise Dead 4, Soul Power 1, Divine Justice 1, Surge of Faith 2

but the hybrid build doesn't have the item support to be competitive. there's a huge tension between stocking up on Health+Mana (totally necessary for Penitence build) and stocking up on minion boosters (totally necessary for Raise Dead build). 

so you just get a build that has all these awesome skills but either the minions are weak or you're constantly low on life and out of mana, depending on which items you had to skip. you may eventually be able to afford Gloves of Fell Darkur AND a Heart of Life but you're not gonna be able to get both any time soon. similar tensions come up between doing things like buying a good armor piece like Groffling Warplate vs. upgrading to rank 4 Idols. not gonna have enough money to do both. 

so again, its not the skills that limit you its the items. 

When you have 6 level 4 minions and nearly a dozen or so spirits surrounding you in a creep wave and with high enough Divine Justice your creeps even stay alive you can really push like crazy, especially if you have a UB teamate along for the ride in the wave to take on any DG's one on one with you assisting.

But like others have said, there are several builds for each DG and they play to several play styles.  But I would not recommend trying to make a "jack of all trades" build as you'll just get owned at the higher levels.

level 1 priest (monks) costs 900.

level 4 priest (bishops) cost 2700.

about 3 times as as expensive, not 4 to 5. 

If you play a general without minions = Fail

 Oak's spirits alone in my build do +600 damage on top of Oak you only need 1 point in pent to interrupt and 1 point in shield to save your butt.

The build I use with Oak is:

Lvl 1: Penitance.  Scares people (except UB) out of your lanes to let you level.  Oak's best power hands down.

Lvl 2: Death Ward.  Mooks are good.

Lvl 3: Divine Justice.  Health and mana are better.

Lvl 4: Pen II or Death Ward II.  Depends on what I'm facing.  A lot of assassins and I'll take pen, generals death ward.

Lvl 5: Surge of Faith I.  Oak's second best power, and really speeds creep farming.  At this point I feel comfortable fighting most demigods, but I'm still pretty cautious with assassins.

Lvl 6: Pen II or DW II (see lvl 4)

Lvl 7: Pen III.  At this point I want to see Demigods running from me.

Lvl 8: DW III.  Spirits are a major part of my DPS at this point, and often surprise people by how much I put out.

Lvl 9: Soul Power.  I've got enough spirits for it to matter at this point, and the boost will be a bit of a surprise.  I can match virtually every DG in straight melee by now.

Lvl 10:  Pen IV.  Now you can't even run.  800 burst damage to lead and finish a fight plus slows.

Lvl 11: DW IV.  Full power to minions.

Lvl 12: SoF II.  Again, basic power increase to a great power.

Lvl 13: Morale I.  My minions at this point match me in DPS, and nobody else on the battlefield is close without artifacts.

Lvl 14: Save.

Lvl 15:  Frenzy and SOF III.  If I can't drop every enemy DG in a straight melee by now, something has gone dreadfully wrong and they've outmoneyed or outleveled me.

Lvl 16-20:  Morale it up, fuzzball.

By the end, without artifacts, I'm slamming down a ton of DPS.  The weakness of this build, of course, is someone who stacks armor (not health, I'm a damage machine by now) because all my damage except Pen and SOF is physical.

I am not sure it's Oak's best build, but it has worked for me.  I always purchase monks early, speed and more speed, and minion damage equipment.  The best thing to do is run from a demigod while assigning your minions to attack him, so he chases you for that kill, but ends up getting rocked by the unexpectedly large minion damage.

1:  Monks

2:  Boots of Speed

3:  Vlemish Faceguard

4:  Plenor Battlecrown

5:  Hauberk of Life

6:  Fell-Darkur

7:  Rest of minion idols

8:  Artifacts that boost minion damage if it goes that far

Actually, most of my money tends to go to cap locks and tele scrolls early after 1-3, so it'll be pretty late that I get any of those, but Oak is one of the demis who has good natural health, so I can generally handle myself.

Switch up to suit situation and enemies (UB I'll tend to take more life items, QoT I'll generally get more minion damage, etc)