The original diocese was founded about 970 by Emperor Otto I in the Billung March at Oldenburg in Holstein (Aldinborg or Starigard), the former capital of the pagan Wagri tribe.
After the Saxon count Henry of Badewide had campaigned in the Wagrian lands east of the Limes Saxoniae in 1138/39, a new Bishop of Oldenburg, Vicelinus, was appointed in 1149.