By the 15th century, some eastern regions of Slavonia were inhabited by Serbs, who settled there after fleeing Bosnia, even before the Ottoman conquest in 1463.

In 1438, pope Eugene IV (1431–1447) sent the inquisitor Giacomo della Marca to Slavonia as a missionary, with instruction to convert "schismatic" Serbs to "Roman religion", and if that should fail, to banish them.

Serbian despot Pavle Bakić fell at the Battle of Gorjani in Slavonia (1537), defending the region from the Ottoman Turks.

[3] Since the renewal of the Serbian Patriarchate of Peć in 1557, the Orthodox Serbs of Lower Slavonia were placed under jurisdiction of the Eparchy of Požega, centered at the Orahovica Monastery.

[4] In 1595, Serbian Orthodox metropolitan Vasilije of Požega moved to Upper Slavonia, under Habsburg rule, in order to avoid the Turkish oppression.