I am flying into Stockholm and will be there for 3 night before heading to Copenhagen for 4 nights. I was wondering if I had time when in Stockholm to take a day trip to Oslo. I didn't know if there were day trips or would I have to use 1 of my nights in Stockholm to stay in Oslo. Any itinerary ideas?
Stockholm to Oslo is a 5 hour journey by train, or probably 3-4 hours by plane if you include getting to and from the city centre to the airport, checking in etc, so a day trip is not feasible. You could spend the night in Oslo and travel to Copenhagen by plane or the overnight ferry.
As Morten says, it's not feasible except if you add an overnight. However, I would not recommend cutting your stay in Stockholm if you only have 3 nights there. I think Stockholm merits a minimum of 3 nights.
I would not try to squeeze Oslo into your 7-night itinerary. If you feel really compelled to do that, fly Stockholm to Oslo, then go directly from Oslo to Copenhagen (by plane or overnight ferry). But you will end up short-changing all three cities.