There are several alternatives when it comes to public transport in Oslo: Buses, trams, metro (T-bane), local trains and bicycles are all efficient ways of getting around the city.
Information on public transportation is available at the Ruter’s Customer Service Centre and Service Points (public transport in Oslo and Akershus) (ruter.no) and on its website (ruter.no).
Single tickets and travel cards can be used on all public transport within the city limits, including NSB local trains and boats to the local islands, Bygdøy and Hovedøya, see overview of tickets and fairs (ruter.no).
Tickets and travel cards can be bought at ticket machines located at all stations and stops. Travel cards and transportation passes can also be bought at 7-eleven shops, Narvesen kiosks and at major subway stations (e.g. Central Station, National Theatre, Majorstua).
You can rent electric bikes and scooters all over the city from different companies. How to get around on Oslo City Bike.