The nearest airport is Oslo Gardermoen. From there you can either take the express train ("Flytoget") or the regular train to Oslo. The express train is nearly twice as expensive and does not go much faster, but it usually departs more frequently and may have more available seating. For purchasing tickets, they each have separate ticket machines and separate mobile apps (Flytoget for the express train, Ruter for the regular train).

The best station to get out of is called "Nationaltheatret". From there it's only a 5-10 minute walk to where the ferry to the island departs, which is called "Aker Brygge". Ferry tickets can be bought with the same app as the train tickets, it is called Ruter (you can use it to buy one ticket all the way from the airport to the island). The ferry takes about 90 minutes to get there.

If you prefer less time on a boat, you should get out at Oslo S (the central train station). Then you can take bus 500 from Oslo Bussterminal, which is next to the station, to Drøbak. Make sure to get out at the stop called "Grande". From there it's a 5 minute walk to where the short ferry to the island departs, which is called Sundbrygga. The bus ticket can be bought in the Ruter app, but the short ferry tickets are sold separately at Sundbrygga.

If you're going by car, you can drive to Drøbak and take a short ferry to the island from there. The ferry leaves from Sundbrygga.

The closest parking lot is Husvik (called Oscarsborg APCOA on Google Maps). You can pay through the EasyPark app, or with one of the machines at the parking lot. There is also a bigger parking lot at Bankløkka.

If you arrive at Oslo S (the central train station) by train, you have two options:

Option 1:

Take the ferry from Aker Brygge all the way to the island. To get from the central station to Aker Brygge, you can either walk through the city (20 min walk) or take tram 12 towards Majorstuen and get out at Aker Brygge. If you take the tram, you can purchase the ticket in the same app as the ferry ticket, the app is called Ruter. It is possible to buy one combined ticket from the station to the island.

Option 2:

Take bus 500 from Oslo Bussterminal, which is next to the station, to Drøbak. Make sure to get out at the stop called "Grande". From there it's a 5 minute walk to where the ferry to the island departs, which is called Sundbrygga. The bus ticket can be bought in the Ruter app, but the short ferry tickets are sold separately at Sundbrygga.