One way of getting around Oslo is to use the citybikes. They cost NOK 49 for a 24 hours subscription and NOK 99 for a 72 hours subscription. You need to download an app to your phone. You can register and unlock a bike with a foreign phone number. The only thing you have to do is to manually select your country code.
You are allowed to keep your bike for 45 minutes before placing it back into a station. Extended rental is available for NOK 5 per 15 minutes.
Read more about how it works here: oslobysykkel.no/en
Station map: oslobysykkel.no/en/map