Originally a French province, it was deparmentalised on December 9, 1848. Its principal town, Oran, was made the prefecture of the départment. There were three French départements in Algeria: Oran in the west, Alger in the center, and Constantine in the east. Oran covered 67,262 km², encompassing five sub-prefectures of Mascara, Mostaganem, Sidi-bel-Abbès, Tiaret and Tlemcen.
It was only in the 1950s that the Sahara was annexed into departmentalised Algeria, which explains why the département of Oran limited itself to what is the northwest of Algeria today. However, prior to departmentalisation, the two eastern territories of the south were managed by the department of Oran.
In 1956 an administrative reform aimed at taking into account the strong demographic growth in Algeria split the province into four parts, Oran, Mostaganem, Tiaret and Tlemcen, taking place on May 20, 1957.