I would agree with the above re: Christmas travel and pricing, so nothing further to add there.
But perhaps some useful SAS/OSL info as I fly regularly there and with them. They often have quite cheap Plus fares, but not sure it gives you lounge access in London. If it's only 30Eur or so more I'd go for that, and on way back you do have lounge access in OSL as well as fast track security. I find SAS generally quite good on European short hauls.
Oslo airport has an abundance of hotels around, and there are shuttles from the cluster just to the West of the airport itself (but paid shuttles so factor that in - around 7Eur each way). Another option for accommodation is Lillestrom, which is about 10 mins from the airport by the regional train. Tickets are about 8Eur and the hotels are cheaper than Oslo city centre usually. This depends on how early your morning flight leaves OSL, as I think the first train is 5.05am, and then the next and more regular ones start at 6am.
Enjoy the trip.