I agree with SK989 90 mins is more than enough, official MCT intl->intl is 30 mins at ARN I think.
With 90 mins between flights you'll even have time to reclaim and recheck your luggage in case US refuses to interline.
Generally it's always a good idea to have some marginal when travelling on separate tickets in case of delays, but this sounds like a viable solution.

Passport control is at ARN and customs in OSL as mentioned. You will have to go through security again at ARN though, if your luggage has been interlined to OSL there is a transfer security checkpoint to the right after passport control which will take you directly to the main transfer hall.