One operator, one ticket. Ruter runs the metro, trams, buses, local trains and the fjord ferries, and they all take the same ticket.
The ferries are the bargain. The boats from Aker Brygge to the islands and the Nesodden crossing are ordinary public transport on an ordinary ticket. A 24-hour pass buys a day of island-hopping at no extra cost.
Buy before boarding, through the Ruter app or from a machine. There are no barriers; inspectors board regularly and the fine is large.
Zone 1 covers the city. Gardermoen airport is not in zone 1 — a city pass does not cover the airport train.
The T-bane has five lines through a common central tunnel. Line 1 to Frognerseteren climbs to 469 metres for Holmenkollen and the forest; line 5 to Sognsvann ends at a lake.
Trams: 12 for Vigeland park; 11, 12, 13 for Grünerløkka; 13 or 19 for Ekebergparken.
Bus 30 for Bygdøy and the museums, all year.
The airport is 19 minutes by Flytoget or 23 by the ordinary Vy train, on the same track to the same platform, for about half the price. See Oslo Airport to the city.
Walking covers most of the centre — Karl Johans gate end to end is fifteen minutes.
The Oslo Pass pays only if you do three or more charging museums a day. A great deal of Oslo is free; do the arithmetic first.
Do not drive. Toll ring, congestion charging, and on-street parking largely removed from the centre as policy.
The full version is getting around Oslo.