There's certainly no problems in driving in one day from Bergen to Oslo, but for me, not being a native speaker of English, a daytrip from Bergen to Oslo (as opposed to a day's trip) implies going to Bergen and back again in one day. Actually I have seen packages offering exactly that, but I wouldn't recommend it! Even so, if the OP doesn't plan on stopping anywhere between Oslo and Bergen, I'd recommend the train.

Obviously one doesn't come to Norway primarily to partake of urban enjoyment, but to be fair, it must be remembered that during the main tourist season in Norway (June 1-August 15 +/-) cities and towns are very quiet as the locals are away. Nightlife, concerts, other cultural happenings are mostly on when the locals (including university students) are in town. And by then the tourists have left....