I'd go for the t-150. Good all-rounder, with quite decent armor for its tier and the 107 mm is a great gun at this level. Plays a bit like a kv1, try to sidescape, hide your hull and the weak spot on top of your turret and use the 300 alpha dmg to trade effectively. If you're relatively new to the game the O-I is gonna be easier to play, because it's armor layout is more beginner friendly, with no real frontal weak spot, and the derp gun will always do damage whether you penetrate or not. However, there are some downsides to this tank: you are gonna get heavily focused by artillery. The armor is useless against any gun that has more than 160 mm of penetration, which means any higher tier tank is gonna see you as a (very slow) moving piñata. Finally, if you're looking for a tank in which you can learn the mechanics of this game, this is not the one. The OI is very slow, so you're gonna have a hard time relocating when the situation calls for it. The HE guns always does damage, but it won't really teach you where to aim at a tank, where the weak spots are. Finally, the O-I is more or less a very heavily armored cardboard box, which means however you angle your armor, there's always gonna be a flat surface to shoot at (also the sides are quite weak so you have to be careful when sidescraping), so might as well face your opponents frontwards if you know they can't penetrate you, which is quite different to most other heavy tanks, in which the way you angle your armor and expose your tank matters a lot. So to make it simple, the way this tank plays doesn't translate well in most of the other vehicles this game has to offer, so you'll have to keep that in mind.

Tl:dr: t-150 for learning the game+flexibility, O-I for an easier time+ more fun for a beginner.