Looking at the schedule that
"Plan & book" brings up, it's a very long day doing a one day return from Oslo. I have to admit that it's easier from from Bergen, and that's the trip I've done, although the train back was delayed by a landslip.
The link says "The tour may be completed in one day, but we highly recommend that you take your time by adding overnight stays and activities along the route", and now I see why that is. The website offers some alternatives for a longer trip.
I was lucky in that we had a local to advise us. We bought the ticket locally rather than online and got hold of a complete timetable to plan our trip. It's a good way to get out of town and see some of the country. The local's advice was that "Once you have seen one fjord, you've seen them all". I have no idea if that's true, but
Nærøyfjord is meant to be a
good one.
Sorry if I raised some expectations that cannot be realised.