I don't have direct experience for OSL, but mid-late December understandably starts getting busy / is in high demand because of peak Christmas travel.
I did an ex-EU from FCO about a month ago (20/12) and booked the long haul almost 9 months in advance. The flights from LHR-FCO never got cheaper, only more expensive, BUT I had a restriction that we had to fly on the last flight of the day due to work commitments -- YMMV.
If I didn't need the TPs to maintain/attain status (i.e. if they were just crediting toward lifetime) and all other things are equal (baggage, flight timing etc) I'd have a hard time justifying double the cost, especially if there's any others on your itinerary. If you have status w/ BA (your profile says blue) which gives you extra checked bags, lounge access or priority, and they are useful to you then obviously they have a non-monetary value that only you can define. I'm lucky enough to have lounge access in either case and so would prob fly with SAS esp if I was travelling with someone.
If I didn't have status, I probably wouldn't pick BA in economy over another full-service carrier unless I was gunning for a particular status level.