train between Oslo and Bergen

National train website:
https://www.nsb.no/en/frontpage

You will probably want to start from the Oslo S central train station. The NSB only books tickets several months in advance. American credit card (chip and signature) may be an on-line problem. See: https://www.nsb.no/en/our-tickets

NSB uses 3D Secure for all our card transactions, whether you buy your
ticket at nsb.no or using our app.

You can quick and easily buy tickets on nsb.no by using the Travel
Planner on the right. You can use Visa, PayPal, Mastercard and Diner
cards. We recommend paying with PayPal for our customers from the
United States, Canada and Australia. Credit cards from these countries
often lack a security protocoll used for safe payments in Europe.

Should you experience problems with any card purchases, please contact
your card issuer / bank. Alternatively, call the NSB Call Centre on
+47 61 05 19 10.

Minipris tickets are the discounted advance purchase tickets.

You may also want to consider the Norway in a Nutshell route (Oslo-Myrdal-Flam-Bergen:
https://www.norwaynutshell.com/original-norway-in-a-nutshell/

I've done the Hurtigruten all the way to Kirkenes and it's a fun way to go. There's a nice waterfront hotel in Kirkenes. Air connections back to Oslo are easy.
If you don't want to spend the time to go all the way to Kirkenes, you can take the ship from Bergen to Trondheim and then train back to Oslo. It's also possible to do the Oslo-Bergen nutshell combined with the Hurtigruten, and if you want you can even buy that as a package and avoid having to make your own reservations:

https://authentic-scandinavia.com/variants/13/map OR https://www.norwaynutshell.com/

I'm planning on doing the Bergen-Oslo Nutshell next summer, and will probably book the transportation as a package because it looks easier, and not much more expensive, than making all the individual transportation reservations on my own.

Definitely book train seats as soon as you can - the minipris seats are a deal even in the equivalent of 1st class - totally worth it! If you go from Oslo to Bergen sit on the left-hand side of the train for the good views!
When we booked our tickets I looked up seats online and then called to buy my tickets. My American credit card was not accepted on the website. There are English-speaking agents to talk to...it was very easy!

BTW the Bergen Line (Oslo to Bergen) has a mandatory seat reservation requirement. At times the trip is capacity constrained. Book early.

Thank you all for your responses. We have been so busy here that I've not had time to make any more plans. I may have more questions once I get started. Thanks again. Julia

I see that train reservations can not be made until 90 days before the trip. Is this correct? Julia

Yes, NSB website allows booking only about 3 months in advance.