I've only been to one ROCOR church, but this is what I observed.
what are the differences between an OCA Church and a ROCOR Church?
No pews; just chairs on the side- greater freedom of movement and a more encompassing worship space. Before the liturgy, I noticed people lining up to talk to the priest, in what I assumed to be a quick confession. I haven't seen this done in OCA churches.
how much of the liturgy will be in Russian?
Probably nothing except maybe for the sermon. The rest will be in Slavonic or possibly English.
should I go to confession there the night before communion?
If the priest doesn't know you, you should ask him. ROCOR seems to have a stricter 1 confession : 1 communion practice. But maybe you should talk to your own father-confessor first.
lastly, my priest uses the traditional OCA service book. our new priest, when he came to visit, used a service book from the ROCOR. I thought this was interesting. is it normal for OCA Churches to have ROCOR-style liturgies?
I don't know, but I think the ROCOR translations are better overall. What is a "traditional OCA service book"? They seem to keep changing things. St. Tikhon's has improved their translation but I think ROCOR still has better texts for the psalms.