Osijek is a city in Croatia.
Go along river from the city centre and just next to old city and fortress (Tvrdavica) there is a bridge over Drava river. The bridge is a part of big road - E73 coming from Bosnian border crossing Samac and Dakovo town in the south to Beli Manastir and border with Hungary in the north. If you go across the bridge there is a café on the right with small parking in front of it - hitchhike right from here.
SOUTH-WEST to Bosnia(Banja Luka)
Going from Osijek to Bosnia toward Banja Luka. You could try to go on high way(E73) that leads back from Osijek to Zagreb just before pay tolls, try to catch a ride to Nova Gradiska and than you are not so far away from Bosnia(driver will probably leave you just before border). On this high way from Osijek to Đakovo it is not so good traffic and cars are going very rarely so maybe it is better to take normal road to Bosnia but you could try your luck(later when pass Đakovo and you drive at A3 high way there is much better traffic). People probably do not use often this high way because you must pay fee for driving there and normal road is free.