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 20 May 1957.