We studied the optical and electrical properties of ytterbium monoxide (YbO) epitaxial thin films with unusual Yb2+ (4f145d0) valence grown by the pulsed laser deposition method. The narrow bandgap of 0.25 eV and the large crystal field splitting of 5d orbitals in YbO determined from absorption spectra were consistent with the chemical trends of ytterbium monochalcogenides (YbChs). Electrical resistivity was tunable upon electron doping via introduction of oxygen vacancies. Electron mobility at room temperature increased up to ∼13 cm2V-1s-1 with increasing electron carrier density. The heavily electron-doped YbO showed weak antilocalization at low temperature, suggesting significant spin-orbit coupling owing to the heavy Yb nucleus.