I have to agree with the Original poster. My brother and I both play Oak in a very similar style, I have lost one online game so far which was a 2v2 when I still won damage and kills but my team mate was an utterly hopeless Torch Bearer who thought he was above citadel upgrades. My brother was ranked 239 in the pantheon yesterday.
When I start the game, 2 Scrolls of Teleport and a Scaled Helm to keep my mana up.
1st major purchase is the bishop idol and that is the only idol I ever buy in most games, unless it's a siege idol in 1v1 conquests.
Item Purchase order: Sell scale helm buy plenor's battle crown, slayer's wraps, Groffling warplate (sold later for godplate), Mageslayer. Cloak of Flames. After these items, usually before cloak actually, I'm dominating so will go for the comedy items like All Father ring or the 20k axe thing. Buy boots of speed after mageslayer if them getting away is a consistent issue.
Skills: Penitence to max as soon as each level is avaiable, Soul power & Ward of the Dead to max, one rank of the kill heal thing (think it's called purity) in the first 3 levels to gain some impressive lane staying power when coupled with bishops, the AoE to Max, then shield to Max.
They key to Penitence is it's timing. It is a hilarious skill: Instant cast, ranged, good damage, small cooldown, medium mana cost, slow, damage buffer, cast interrupt. You can use it to harrass but if you're going into melee to attack someone do not use it straight away. The key to demigod is skills, they are the big killers, and most of them have a cast time, it takes practice but once you've seen all the spells being cast, knowing when to fire is just a case of quick reflexes. Most players will stay in combat or meet you head on because they have one skill that they believe will trump you and give them the advantage, be it Rook's Hammer, TB's freeze nova or Erebus's Mass Charm. They will either use these skills to try and stand toe to toe with you or to start retreat, once interrupted they are taking more damage, badly slowed, and left floundering with their skill on cooldown and no way to fight back, or their plan of escape completely ruined.
Swift Anklet Favor item, debatable whether or not the 2k favor blink cloak is better in all honesty, the only situation I can see it being better is 1v1 vs an Ice Torch bearer.