Even though there is only one harbour specially designed for guests, there are more than ten boat clubs offering available berths to visiting boats.
You will find everything from harbours in the city centre to quiet places on an island in the Oslo Fjord. Click on the link, and you will find a survey of the harbours which offer berths, the facilities available in each and their contact details (scroll down the page for information in English, French and German).
Located near the centre of Oslo:
Herbern, Aker Brygge
Frognerkilen, Skarpsno, Skøyen
Lille Herbern, Bygdøy