Oak is a peculiar mix of paladin and necromancer, although this assessment is debatable.
He can raise the spirits of the deceased (enemy and ally alike) to fight for him, heal himself and his allies by killing enemies, shield himself or allies completely from damage for a short time, make enemies more suspectible to battle damage and your own folks (dead or alive) stronger, faster, deadlier. A necromantic Paladin General.
There are various ways to play an effective Oak. There are and will be many different builds telling you how to make your Oak powerful. Demigod however is not a static game where one build rules them all. The power of a build depends on the objective of your current map (conquest, fortress, etc.), your teammates and, of course, your opposition. This is true for any Demigod.
This being said I present you with a guide that can serve as a, um, guide for an effective Oak.
Groundworks (Level 1-11)
The following skillpoint distribution is what I consider the bare minimum of what a spirit using Oak needs. You can decide midgame in which direction your Oak goes after this.
Most build writers like to give a skill point distribution order with their guides. I noticed that I don't even follow my own distribution order when actually playing, so scratch that. But some priorities are in order:
I usually start with raise dead, followed by penitence. Makes enemies keep their distance. I follow up with Divine Justice to keep my minions alive and kicking, then I go for Raise Dead 2. The rest depends on the game, really, although I tend to make Raise Dead a priority.
Raise Dead 4
Oak can summon spirits by planting a banner near dying units. Oak doesn't even have to stay there like Erebus - planting banners around the battlefield is enough to harvest the spirits of the dead. Spirits can become quite strong and resilient and they strenghten Oak himself who in turn strenghtens his spirits by healing them when he kills a unit. Every Oak build should take advantage of these cost effective minions. A guide describing an assassin type Oak neglecting Raise Dead for the first few levels can be found here.
Soul Power 1
No matter what you do, Oak will find himself in the thick of the battle and Soul Power will increase his damage significantly even with just 1 point invested. For an in-depth analysis of the Soul Power mechanics see this thread.
Penitence 4
You want your enemies dead and Penitence is one of the game's best way to do it. The highest level of Penitence deals 800 damage, reduces enemy movement speed by 16% and increases all damage taken by that enemy by 16% for 7 seconds, which is exactly the cooldown time for that skill. No matter if you, your minions or your allies klonk him ... your opponent will feel it.
Divine Justice 1
Divine Justice gives you and your allies health and mana for equal to 10%/15%/20% of every target killed by Oak himself. Rally also adds 500 armor and 1000 health. The first level of that skill is the most cost effective, though, and I tend to leave it at that.
Shield 1
Shield makes you or a target ally completely immune to damage for a couple of seconds. Higher levels remove debuffs or even heal the target, making this an extremely valuable skill, although the most effective use requires either experience, good reflexes or a strategy.
This skill will save your demiass more often than anything else: Shield up, run away. Shield up, teleport. Shield up, drink potion. Shield up, Heart of Life.
Mid- to Endgame(Level 11-20)
Depending on your primary objectives and the way the battle goes you can choose on how to distribute further skill points. I offer two distribution plans, each suited to a different goal/situation.
The skills listed here are taken in addition to the skills covered in the groundworks section. They whole point of having a common base skill distribution is the tactical flexibility of developing your Oak depending on the tides of battle.
Spirit Assassin
Your primary goal is to kill other Demigods and up till now, you fared well enough. The Spirit Assassin further enhances the power and toughness of his spirit army, leaving the killing mostly to them. Farm the lanes and sucker your opponent in, shove a penitence down his arse, then sick your army on him. Since spirits can fly they will have no problems reaching other DGs, even with a big army in between. Learn how to steer your spirits independently from the rest of your army (should be the hotkey "I").
Soul Frenzy
Useful for any build. Hits your spirit army into a frenzy, increasing their movement speed by 25%, making it quite hard for them to flee, especially after being slowed by penitence.
Surge of Faith 3
Oaks only AoE skill. It deals 600 damage to all enemies around him and - here comes the good part - increases attack and movement speed by 20% for all allied units, making it even harder for enemies to flee.
Morale 5
Each level of morale increases your minions health, regeneration, armor, damage and attack speed by a small amount. Take this skill up to level 5 and the amount is quite noticeable.
or
Morale 2
Divine Justice 3 + Rally
While the first point in Divine Justice is the most cost effective one, maxing DJ and getting Rally is an advantage all of it's own. Just consider this scenario: You and Unclean Beast beat each others brains out and UB decides he has enough. You follow him, finish him off ... and stand dumbstruck between two Towers and a freshly spawned mob, armed to it's teeth. An extra 1000 health and 500 armor in addition to 20% of UBs health would be awfully handy right now.
Juggernaut
Your primary goal is to kill fortresses or the enemy has some well guarded buildings that really need leveling. Or you just die too often and start to show signs of frothing. The Juggernaut increases the staying power of himself by ridiculous amounts, enabling to march right through an enemy army, go for a fortress and smash it to rubble while completely ignoring armies, Demigods and inconveniences like death.
Shield 4 + Purity
The highest level of Shield combined with purity protects you from all damage and all negative effects by 6 seconds, which is plenty when you focus on increasing your melee damage. This can either be a live safer for you or your allies, or it's the beginning of the kamikaze run on the enemies base.
Soul Power 3
Since you are the (temporarily) unkillable one, it makes only sense to increase your damage output as far as possible.
Last Stand 2
You are dead - so what ? After technically dying you can still move around for 10 seconds ... but with attack, movement and damage increased by 50%! Either klonk the one that did you in or - which makes this skill nearly overpowered - head straight for the former impenetrable defenses. Nothing can kill you before your 10 seconds are up, so put them to good use. Using this skill, fortress games are a breeze. Power up your shields, head straight for fortress no matter how well defended, bash it, die, bash it more.
See this thread written by a victim of an Oak Juggernaut.
Soul Frenzy
Well, you got one point left and this is a pretty decent passive skill, so you might as well grab it.
Items
I usually focus on Idols, a bit of mana boosting and stuff enhancing either melee damage or minion strength, depending on the direction I follow. Suggestions are appreciated. 
And just for fun some gameplay videos of Oak in action. I sadly wasn't able to reach higher levels before trashing the enemy, though.
