3 days in Oslo and transport to Copenhagen

Thoughts on whether to go by train or plane?

Ferry. Or if you have the time, by train and stop a few days in Gothenburg.

Yes— the ferry is a relaxing alternative to get from Oslo to Denmark.
www.DFDS.com

Interesting - I never thought about the ferry. Will look into it. Thanks.

Sharing some favourite things in Oslo- Viking Ship Museum (there are also some other ship related museums nearby, like one with the rebuilt Contiki), Vigeland Park (cool sculpture park), and the National Museum (I believe free on Sundays). You can also visit where they give out the Nobel Peace Prize if that's of interest. I've been to a lot of places and Oslo is by far the most expensive place I have ever been, so I was very conscious of doing things that were free! Just be prepared for the prices- Copenhagen will seem inexpensive in comparison.

The ferry is good. I believe the Viking Ship Museum is closed for a long term rebuilding project, but there is plenty else to see in Oslo. And you can see Viking ships in Roskilde near Copenhagen, although they are not as good as the ones in Oslo that aren’t currently viewable.

Sharing some favourite things in Oslo- Viking Ship Museum

This is closed until 2027. Take the one next to Roskilde, day trip from Copenhagen.

coffee, pastries,

For tasteful more a Danish thing called Plunder (link o picture). In Oslo you would like "quaint neighborhood" Grünerløkka.

local history

Norsk Folkemuseum and / or Museum of Oslo

nature, hiking

Holmenkollen

Ohh that's too bad about the Viking Ship Museum. Agree that the one is Roskilde is a good back-up option, tho those boats are less well preserved. You can combine it with a visit to the cathedral where the kings and queens of Denmark are buried. The Fram Museum is still open if you did want to see boats in Oslo: https://frammuseum.no/

MUNCH is a must for those who like Munch's artwork.

In addition to the places already mentioned, I liked the small Arbeider Museet (Labour Museum) sitting along the Akerselva River north of the center of town. I'd call it a social-history museum. Caveat: I have a high tolerance for reading written material in museums. https://www.oslomuseum.no/en/plan-your-visit/labour-museum/

You'd have to be lucky to be in town when it's open, however; until June 19 it's only open on Saturday and Sunday.

If you decide to take the ferry, you can also book through Booking.com. When we decided to take the ferry, I tried emailing the addresses listed on their website--to no avail. I had several points I wanted clarification on, based on info listed on their website and comments I had read from other folks who took that cruise. I never received a response. When I saw that I could book viat Booking.com, I messaged the company and received a prompt response. FYI my questions had to do with free cancellation (yes) and whether the Commodore room we planned to book would be near the disco (no). I think the rate was slightly cheaper than the rate I saw on the ferry website.

Thanks everyone. I appreciate the thoughts on things to do in Oslo.