There are a few methods to get from Oslo to bergen:
1) Take a flight
A plane journey from Oslo to Bergen takes under one hour and there are over 20 flights every day Traveling between the airport and the city center is easy (Bergen Light Rail Bybanen or the airport shuttle bus Flybussen).
2) Oslo to Bergen by train
This trip will take around 7 hours. The train passes through spectacular landscapes and gives you a lot of options for excursions on the way ( Flåm Railway).
3) Driving from Oslo to Bergen
Driving by car from Oslo to Bergen takes around 7-8 hours.
4) By bus the journey will take around 9h 30 min. The road from Oslo to Bergen is wonderfully picturesque and gives you a good view of some of the vast and wild nature of Norway.