Slavonia was an autonomous kingdom within both Croatia and Hungary, themselves within the Habsburg Monarchy/Austrian Empire; Croatia-Slavonia, its successor, was an autonomous kingdom within the Lands of the Crown of Saint Stephen (Transleithania), the Hungarian part of Austria-Hungary.

The capital of the county moved from Virovitica to Osijek (Croatian; Hungarian: Eszék) in the late 18th century.

In 1854, as part of Bach's reforms, it was renamed Osijek County (Comitat Essek) and its territory changed: its western areas around Voćin, Slatina and Virovitica passed to Požega County and it absorbed the western parts of the former County of Syrmia around Vukovar.

[1] (The eastern part of Syrmia now belonged to the Voivodeship of Serbia and Banat of Temeschwar.)

In 1920, by the Treaty of Trianon the county became part of the newly formed Kingdom of Serbs, Croats and Slovenes (later renamed to Yugoslavia).