Only way to get to the lounge is to go via international transitt gate to get to the lounge. And that means that you must travel without baggage. Baggage has to be picked up and carried through customs at OSL. When re-entering the departure hall, you will not have access to the international departure area as you dont have an international ticket.

But as you arrive late and the lounge normally closes at latest 11 pm, I dont see why you need to figure out this. Your best option is to book a room at Radisson SAS/BLU which is accross the street from the airport.

BFN