There is a non-Schengen transfer desk. I wouldn’t count on it being open. And assuming you have no issues clearing Schengen, would do that as much faster (cnx point ca 50m from border check). The non-Schengen transfer desk usually requires someone to be called over, some spotty reports on FT of people doing it.

Suppose reference to opposite end also may come from check-in location. Think BA is as far away from the F gates as possible, though think QR somewhat closer to security. Been a while since I’ve used either from Oslo.

And if lounge is of concern, they’re only in the Schengen area.

I’d say you can’t be back at gate 30min after exiting aircraft, probably in 20-25min depending on your walking speeds.