When we hiked Priekestolen, we started early in the morning (staying in Stavanger) took the ferry to Tau, then hopped a bus to the trailhead. Returned the same way. Tickets for ferry/bus combo can be bought day of, at the ferry terminal.
I would recommend bringing a lunch with you to eat at the top while enjoying the view. Bring plenty of water! Closed-toe shoes are a must (hiking boots not mandatory, we did sneakers) as the trail is irregular and rocky to put it mildly. Trail splits in two part way, we did cliff trail up and the other one down, they are unique in their own ways, so see both!
Transportation from Oslo - I defer to the experts, but if you started early enough in the morning (from Stavanger) you could likely make it down in time to catch a night train back to Oslo and sleep on the way. We flew from Stavanger to Oslo, but we had a TATL to catch. Can't comment on how scenic the train ride from Oslo to Stavanger is - might be its own adventure!