The still-extant Diocese of Osnabrück, erected in 772, is the oldest see founded by Charlemagne, in order to Christianize the conquered stem-duchy of Saxony.

The episcopal and capitular temporal possessions of the see, originally quite limited, grew in time, and its prince-bishops exercised an extensive civil jurisdiction within the territory covered by their rights of Imperial immunity.

Klemens von Gruben, titular Bishop of Paros in Greece, was made vicar apostolic of Osnabrück, and as such cared for the spiritual interests of the Catholic population.

The ordinary Latin (Roman) Catholic episcopacy was restored in 1824, but henceforth the bishops would no longer wield any temporal power.

The temporal protectorate (Latin: Advocatia; German: Vogtei) exercised over so many mediaeval dioceses by laymen became, after the 12th century, hereditary in the Amelung family, from whom it passed to Henry the Lion.

In 1236 the Count of Tecklenburg was forced to renounce all jurisdiction over the town of Osnabrück, as well as the lands of the see, the chapter and the parish churches.

He offered little resistance to Lutheranism in Münster, though he vigorously opposed the Anabaptists; after 1543 he allowed in Osnabrück an evangelical service.

His successor, John IV of Hoya (1553–74), was more Catholic, but was succeeded by three bishops of a Protestant mind: Henry II of Saxe-Lauenburg (1574–85), Bernhard of Waldeck (1585–91), and Philip Sigismund (1591–1623).

But in 1633, Osnabrück was captured by the Swedes: the university was discontinued, Catholic religious exercises suppressed, and the see (1633–51) administered by the conquerors.

Today Iburg Castle is a museum and seat of a local court while the Bishop's Palace houses the University of Osnabruck.

King George I of Great Britain died in the palace while visiting his brother, prince-bishop Ernest Augustus, Duke of York and Albany, in 1727.