I have found that being able to survive is key to holding flags early game. I also need the magic repelnishment to limit trips back. I think Soul Power is not that important unless you have a respectful number of Spirits flying around.

I also have shield and penitence to give me the upper hand in damage output in 1v1 before lvl 5.

In a build where you arent supporting your minions with either skills (Morale) or items, it doesnt make any sense at all to be taking 2 levels of Ward but not getting Soul Power until the game is basically over.  You basically have 3 choices:

1) Use SoulPower (maybe only 1 level though) to get a benefit from your spirits

2) Do a minion build with Ward

3) Dont get Ward (or at least not until very late in the game)

You are waiting a long time to get Vlemish for a Penitence based build with Surge.  I dont think thats going to work well at all.  I also find it hard to run a Penitence/Surge build with just Vlemish and need Plenor or a whole lot of mana potions.

I also have no idea why you are waiting until Mageslayer to get your 5th item.  Having an open item slot that long just doesnt seem wise to me.  For example, you could just not sell back Banded armor...  Also I've never found Mageslayer to be a really good value for Oak.  You just dont use it enough to justify getting it over say creeps and whatnot.  Heck, if you must buy it, I'd probably recomment giving it to another teammate.

Why are you taking High Priests over Bishops?  You arent Sedna.

This is a very slow build.  You probably wont be getting many kills with it compared to normal assassin builds which actually have some speed.

Not taking Monks at level 1 is probably a mistake.  Those things are amazing at level 1.

You dont have enough mana to use them more than once or twice before level 5.  Counting on them to do everything for you is non wise.  Soul Power adds a hugh amount of damage to your auto attacks with just 3 spirits out for that early in the game.  And that early in the game is when you will probably be auto attacking the most.

IMO this is actually one of the least effective Oak builds for fighting kiters that I have seen.  You dont have strong spirits to force them off, you dont have as much speed to chase them down.

According to Kestrel its "the best level 1 skill in the game, bar none," but I'm not seeing it. However, its probably the best Oak can do, since all his other level 1 skills are bad. 

Shield 1 early isn't about staying power. It's about saving yourself from bad situations like a fatal snipe, pounce, or other nukes. Using Cloak of Night to get out of AA range, then shielding any ranged nukes afterwards (along with removing debuffs like slow) gives you excellent survivability early game.

Oddly enough, at 15 this build is identical to what i have on Oak.  The progression, however, looks bonkers to me.  There's no focus.

At level 10, I have 4 Penitence and 2 Surge.  I decide whether the current matchup calls for me to focus on support or damage.  If i'm support then i take 3 Shield and 1 Divine Justice.  If i'm damage then i take 3 Spirits and 1 Soul Power.

After 10, grab the 4 points in the other half (damage or support) that weren't taken previously.

I have tested this build in a few games today. I LOVE it!! I had to change the items to buy like this:

1. Banded Armor
2. Monks
3. Velmish.
4. Boots of Speed.
5. Nimoth without selling banded.
6. Wand of Speed.
7. Slayer Wraps.

I am calling it Oak the RoadRunner

Cloak of Night is a huge ambush item and a perfect escape method. No one expected me to use it and when I did the actual player not the DG gets stunned!!! After getting boots of speed and Wand of Speed, I can follow running DGs into their citadel to finish them off and I can run across cataract in no time. I have routed quite a few Oaks (Assassin and minion builds) and scared off TBs and Regs not just to their tower but even all the way back to their base.

Its true that I don't rack many kills in the beginning. I am a late bloomer who once has the speed set (Boots+Wand) becomes inescapable.

This build does not follow the standard builds people have started using and players learned how to counter. This has surprised some UBs, Oaks, TBs and led them to an untimley death.

That is very similar to what I used to do. I have found that people tend to learn to counter this build really quickly. I have turned it around a bit and then changed the favor item and armors to even screw with people heads even further. I start as survival and support than add the speed gear to become lethal in a way that they do not expect.

Without Shield, you will be easily pushed back at lvl 1 a single spit, fireball or pounce can push you back but if you take the hit and then spend three seconds retaliating, thats a different story.

I think you're confused. High Priests are the Rank 3 idol, and are "bugged" with Healing Wind to heal for 30%. Therefore Sedna always gets High Priests. Bishops are the Rank 4 idol. They are the best one sans Healing Wind, which is why other generals should usually get them.  

Also, Monk idols are unbelievably strong at level 1, and in fact counter all the things that you apparently are worried about in your reasoning for taking Shield at level 1.