Perovskite-oxide materials have grabbed enormous attention from various research groups all over the world due to their large application areas. The band-gap engineering of those materials are important for optoelectronic researches especially for ferroelectric (FE) solar cells that have unique features such as having higher open circuit voltages than the band gap and their spontaneous polarization which leads to photovoltaic effect. Nevertheless, the most of the perovskite FE materials have wide band gaps that hamper the absorption of large solar spectrum. In the present study, it has been demonstrated the band gap of YMnO3 (YMO), which is one of the mostly studied FE materials, can be tuned via doping osmium (Os) into manganese (Mn) site. The band gap of YMO, 2.10eV successfully is lowered to 1.61eV. Polycrystalline YMnO3 and YMn1-xOsxO3 (YMOO) (x=0.01, 0.05, 0.10) thin films were synthesized on indium tin oxide (ITO) substrates at 500 degrees C by magnetron sputtering method. Their structural, chemical and optical band-gap properties were studied and the results showed the Os doped YMO compounds could be a potential candidate for future ferroelectric solar cell studies.