Yes, NSB tickets bought online are only valid on the exact train you bought the ticket for. Full price tickets can be cancelled and refunded (minus a processing fee) before the scheduled departure, but if you choose to take a different train, you will have to buy a new ticket for the new connection.

Your question is however a bit odd, since tickets for travel within the greater Oslo area, including the trip between the airport and the central station, are not sold by NSB, but by Ruter, the local transportation authority. Single trip Ruter tickets can be bought at ticket machines, on board the train (in most cases with a surcharge if you could have bought the ticket before boarding) or with the [RuterBillett][1] app, which allows payment by credit card. They don't sell online tickets through any web-based channels and there is really no point in buying the ticket in advance.


  [1]: https://ruter.no/en/buying-tickets/mobile-ticket-app/