With 8 hours to kill I'd be straight off into Oslo. I'm not sure if there's left luggage at the airport but I know for certain that there is plenty of left luggage at Oslo Central train station as I have used it myself on several occasions.

My advice is, collect your luggage, jump on a train to town (takes 25 minutes or so and there's a train every 10 mins), put your bags in a locker and go for a stroll to stretch your legs and get some fresh air. There's plenty to do, see and eat within easy walking distance of the train station.

Just the thought of spending 8 hours at Gardermoen is making me sweat, and it's already been pointed out that the BA check-in is unlikely to let you check your bags in until 2hours before departure.