Would like to get best info on the best train to take from Oslo to Stavanger. My wife and I will go there for 2 or 3 days and then fly home from Stavanger to US by way of Copenhagen
It looks like a day-long journey. Note that some of the options shown here are buses rather than trains:
You can read about Norwegian trains on the Seat61 website. You'll need to scroll way, way down to the "Trains in Norway" section.
That is about an 8 hour train trip. No changes, but a lot of stops and a long day. You may want to breakup the trip and spend some time in Kristiansand.
I'm not sure I understand the question. If you want to go to Stavanger, you take a train that goes to Stavanger. The best one is the departure that best suits your plan. Book in advance for the best price.
Trains between Oslo and Stavanger are operated by Go-Ahead Nordic under the brand Sørtoget, buy your tickets and read more at https://go-aheadnordic.no Both day trains and night trains are available.