I have traveled in Norway on the train network, and found it to be exceptionally punctual.  

However the journey time from Trondheim to Oslo is around 7 hours, and you only need one small problem to crop up and you will end up having to pay for a extra segment to Kristiensand 

The nsb website shows that it is a possible connection, but flags a warning 

Looking at the online time table it seems that you are hoping to catch the 23.13 in Trondheim, for the overnight service, with the connection at 07.25, my suggestion would to be on the safe side and aim for the connection at 11.01. That would give you a few hours in Oslo to get breakfast.  

The standard price of the Oslo Kristiensand journey is 677kr which is a lot to pay for if you miss the connection

Even though I think it is likely that you could make the earlier train, my personal thought is don't risk it.