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.
Tickets for shorter train trips within the greater Oslo area are priced according to the zone based fares of the local transportation authority 'Ruter'. If you order an online ticket from the airport to the central station on nsb.no, you will actually pay NOK 101 for a four zone (4N, 3Ø, 2Ø and 1) Ruter ticket, but it will only allow you to travel on the designated train. If you buy an 'on-the-spot', regular Ruter ticket instead just before travelling, you will also pay NOK 101, but the ticket is valid for arbitrary public transport (also bus, tram, subway or ferry) and transfers within those zones for the next 2.5 hours.
Ruter tickets can be bought at ticket machines (there are plenty of them at the airport), on board the train (in most cases with a surcharge if you could have bought the ticket before boarding) or with the RuterBillett 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.