There are some possible accomodation for your trip:

Hostel Tufna
Osijek, Franje Kuhača 10
from 97 HRK/person/night

Crni Biser Apartments and Rooms
Osijek, Hrvatske republike 26
from 110 HRK/person/night

Guest House Talas
Osijek, Josipa Reihla Kira 14
from 132 HRK/person/night

Hotel Millenium
Osijek, Kralja Petra Svačića 12
from 222,5 HRK/person/night

Hotel Silver
Osijek, Martina Divalta 84
from 280 HRK/person/night

Or you can find more options on the www.booking.com website.