I think Oak is pretty well balanced. A minion based Oak is even more fearsome than the build that you list.
The best way to fight Oak is by getting items to boost your armor and health. I think that the favor item "Symbol of Purity" can remove that debuff effect that Oak puts down, but that does mean that you are giving up other potential favor items that might benefit you more.
Orb of Defiance isn't a bad choice, but abilities like Penitence are up rather quickly. Proper timing would at least provide some safety.
Sadly Penitence doesn't lower your speed enough (even against Rook) to justify the use of Ironwalkers... I suppose it's not terribly relevant, but I figured I'd let people know that they won't be useful here in case someone was thinking about it.
I suppose the best options are very simple: +Armor, +Health, +Movespeed, and slow him down if you can... Of course the alternative is: +Damage and then try your best to slow him down and keep him from escaping. Killing him or making him run away is a viable alternative to having to flee yourself.
Hmm... Also it would be worth noting how much damage he does and how fast he is when playin against him so that you can better determine when to retreat (and not get killed).
As I see it, Oak is the character that the developers should reference when balancing the game. Oak's abilities don't provide all that much benefit when you start to crunch the numbers, but, when you add in another person, abilities like Penitence can become quite fearsome.
EDIT: Although I've never tried it, I think that Sludge Slinger might be very useful against Oak. Oak does decent damage and he attacks fast, and -30% to attack speed should dampen his damage output for a short time. If you fight a minion based Oak then you should pick up the Parasite egg.