hi jongalt26 - that's a good classic Assassin Oak build. ProneG and I have an ongoing debate on whether a build like yours or one like mine is better for tight team games (any thoughts, people?)
Any critiques I might have would be around items, but really, if your selections work for you don't change them..
Some things that stick out -
I would say experiment with getting monks early. If you use them right you don't have to avoid other DGs early and can push most everyone around, even with a hybrid build like mine where the minions don't hit their stride til later. Actually, if you get left alone you can down a tower at level two and walk away with half health...try that with 1000g of armor ^^
Oak doesn't really need Teleport Amulet like some other DGs might (Rook) unless you are doing a bunch of flag-hopping on a larger map. Cloak of Night + Shield allows you to pull out some ridiculous kills/saves, and BoTF means no one can push you off flag solo..
Unbreakable Boots before Nimoth, imo, just because it's so cheap and has huge value
L4 Siege is a Good Call
Also, something to keep in mind is that you can operate an Oak like that for most of a game with a helmet, monks, and Unbreakable Boots without grave risk. That gives you a bunch of money to play around with, so if someone needs to be designated Cit upgrader there are worse candidates
Anyway, see what you think and report back if any of that helps