Area of today's Osijek-Baranja County has been populated since the Neolithic period, the oldest known people from that period being the Illyrians. Celts penetrated the area in the 4th century BC, and erected the mighty Roman fort and town Mursa at the location of today's Osijek, which has been elevated to the status of a Roman colony by Emperor Hadrian in year 133. After many years of devastation of this Roman settlement and numerous barbaric tribes passing through this area, Croats settled here in the 7th century.
In 1526 Osijek and the entire surrounding area were conquered by Suleiman II and his Turkish army, and they remained here until 1687 when they were expelled by the Austrian army. As a part of the Austria-Hungary state since the 18th century, this area has achieved intensive economic development and expansion.
After the World War One, in 1918, this area became united into State of Serbs, Croats and Slovenes, and since 1945 it was a part of the so-called New Yugoslavia.
In 1991 Osijek-Baranja County was the first to suffer the aggression of Serbian paramilitary forces and Yugoslav Army, and almost half of the county territory was not under control of the Croatian state. Through the process of peaceful integration, Osijek-Baranja County has been returned to constitutional and legal system of the Republic of Croatia in its entirety on January 15, 1998.