1. If you haven't booked yet, this fare offers free stopovers. On the return leg, the cost difference would just be the taxes on a Club Europe flight that wouldn't apply on a connection. So you could go to Oslo weeks later, with or without a bag.
2. If you have booked, don't check in for the OSL-LHR return. If it's not the last flight of the day, use the same-day change to move it to the later flight from Heathrow.