A note of caution is the gates BA use are at the very far end of the terminal - so once you clear security (usually 5-10 mins max), you then have a 7-8 minute walk up to the far end. By far the biggest queue / wait time normally faced in OSL is the exit Immigration Control, again at the far end of the terminal after security, just before the gates BA use. The same gates are used by DY for their afternoon USA flights and this means there can be pretty long queues at times when a BA departure is timed close to a DY departure.
My last time I flew out of OSL was 4 months ago.....security took 5 minutes, exiting Immigration Control took 30-35 minutes for the late afternoon BA flight and I only just made my flight!
similar for me in September. No fast track security for BA CE but normal security was a breeze. Exiting passport check took 20-25 mins but a QR was boarding so may not be typical.