Anyway, to start with I went to kayak.com and searched for the location and dates I wanted, trick though is to be as flexible as possible. So I have put +/- 3 days to be as flexible as possible.
I then scrolled down quite far and on the left-hand side under 'Stopover Airports', I changed the time to at least 5 hours, this ruled out the quick stopovers. You then have to unclick the box 'Hide 14,267 longer flights'.
Then back at the top, the first option shows the departure from STN to BOS via Copenhagen using Ryanair first then Norweigan air. For the return, the option showed Norwegian Air to Olso then Ryanair to Oslo. I went directly to the websites themselves (Ryanair and Norweigan Air respectively) and managed to get it down from £428 to £280 by playing around with the number of nights I want to stay in each place. This ended up being 1 night in Copenhagen and 2 nights in Oslo. Perfect for me as I've always wanted to visit Denmark/ Norway!
Remember, you will have to play around the dates to find the best options.
If you have any questions, leave a comment and I'll try to get back to you ASAP!
Happy travels x