He was a member of the House of Kotromanić, most likely son of Vladislaus and brother of King Tvrtko I.
When duke Hrvoje Vukčić in 1416 died, King Ostoja divorced his old wife Kujava from the house of Radinović and married Hrvoje's widow Jelena Nelipčić the next year.
Ostoja was brought to power by the forces of Hrvoje Vukčić, Grand Duke of Bosnia and a Herzog of Split, which deposed Queen Jelena Gruba in 1398.
King Ostoja led a war against the Republic of Dubrovnik, a Hungarian vassalage that year.
In 1404, the Bosnians under Hrvoje Vukčić replaced him by his brother Tvrtko II because of his pro-Hungarian views.