Sightseeing in Norway

You could use the Nutshell tour Oslo to Bergen as and return as your transportation between the two cities. 8/8 outbound Oslo to Bergen and 8/10 return to Oslo.
http://www.norwaynutshell.com/en/explore-the-fjords/norway-in-a-nutshell/

The outbound is a long day of trains, boat and bus, depart morning, arrive Bergen in the late afternoon/early evening. Return is a train ride direct with local stops.

I was thinking along those lines Edgar but I had some doubts about using the Fjord Tour Companies. Do you suggest paying through them or just purchasing public transportation for each leg of the trip on my own?

And since this is summer time do I need to book all this now or buy tickets for trains trips now from the USA or is it safe to buy in country?

Ben,
The multimodal aspect of the outbound Oslo-Bergen includes three trains, one boat and one bus element. Paying the Nutshell agency to coordinate that leg is worth any premium you would pay.

I also believe that the Nutshell agency's ticket allow for coordination of travel. If you book directly with NSB, they have the following transfer rule:
"When a journey involves a change of trains, the transfer time is defined at ...60 minutes for NSB Regiontog (inter-city train) services."

You may be able to economize your return Bergen-Oslo by booking directly with the NSB if you can get a Minipris (advance purchase discounted ticket). The NSB sells a limited number of Minipris tickets, with the cheapest being the most advanced booked. I last visited Norway in 2012 and was able to advance book and pay for Minipris tickets over the web using my American CapOne Visa. Apparently NSB has changed their credit card acceptance and you may need to do advance booking by telephone if your card is not accepted via the web.

https://www.nsb.no/en/our-tickets

Buying your NSB ticket: Either you choose to buy your ticket online, using your mobile, at a ticket machine or at a station, here is how you do it! You can book your train journey using the Travel Planner. Should you experience problems using your debit or credit card, please contact the NSB Call Centre , phone number 815 00 888/+47 23 62 00 00. Press 9 for information in English. We are continuously working on establishing secure payment for our customers. If you are having issues with payment, please contact our customer service or try booking at scandinavianrail.com .

See earlier forum discussion on booking rail: https://community.ricksteves.com/travel-forum/norway/reserving-train-ticket-to-oslo-via-email