If you schedule an hour from gate to gate, you'll have enough time to pick up luggage, check it in and be at at non-Schengen gate. The BA checkin counters are at the far east of the terminal, so make a left when you exit customs and take the lift/escelators upstairs.
Without checked luggage it will take you ten-fifteen minutes from gate to gate. Use the transfer point at gates 42/43 on the arrivals level. Follow "Transfer" signs. These facilities may not be open at late night and early morning.
https://avinor.no/en/airport/oslo-ai...trip/transfer/