If you are travelling from Oslo to Bergen, do take the train and do be sure to reserve a seat in advance, particularly in the "high season" of June-August. The Oslo-Bergen route takes you up and over the Norweigan Alps, and while it isn't a fast train, it certainly is one of the most scenic in Europe.
You can get to Bergen either by plane, car or train from Oslo, ferry from Newcastle or Denmark.
The best way to get around Bergen is to either walk, (it depends on how far you want, of course),bus or by car.
Bergen itself is beautiful - but already the train journey there is spectacular! Especially if you're short on time and want to get a bit of the fjord scenery around Bergen, the trip from Oslo to Bergen is perfect for that!
I mean, I'm from Switzerland and we're here 'spoilt' with beautiful railyway lines, but the 'Bergensbanen' is definitly something extraordinary. Changing weather conditions and the contrasts between the lowlands and the high mountains make it a new experience every time.
By the way, of course most people travel in the summer, but this region is actually more beautiful outside the summer season - when the tourist traffic has quietened down and the experience is even more intense: The landscape is at its most beautiful during the explosion of autumn colours, when the snow covers the landscape like a white blanket or when the buds spring forth in the spring and everything slowly springs to life again...