They still have post offices in and around Oslo? I cannot recall seeing one at OSL airport in recent years (not that I have been looking), but there was at least a mailbox somewhere around there and stamps could be bought there or at the Radisson IIRC.

[In Stockholm, most such services are provided at grocery stores and at convenience stores/newsstand+tobacco-selling type stores -- even the traditional post office building in central Stockholm was sold off some years ago as I found out when trying what used to be normal before for non-residents: request mail to be held at the main post office for pick-up in person by the non-residents.]