It was originally a suffragan of the archdiocese of Hamburg-Bremen, from 1104 on of that of Lund and starting from 1152 on of Nidaros.

It then covered the (modern) counties of Oslo, Akershus, Buskerud (except Hallingdal), Hedmark (except the northern part of Østerdalen), Oppland (except Valdres), Telemark, Vestfold and Østfold, and the province of Bohuslän, and the parishes of Idre and Särna.

The regions of Hallingdal and Valdres were transferred from Diocese of Stavanger to Oslo in 1631.

The Diocese of Tunsberg (with Vestfold and Buskerud) was established and separated from Oslo in 1948.

The Diocese of Borg (with Akershus and Østfold) was established and separated from Oslo in 1969.