Scanning tunneling microscopy (STM) and ultraviolet photoemission spectroscopy (UPS) measurements for oxygen adsorption on Ru(10 (1) over bar0) were performed. The STM images taken on the c(2 x 4)-2O phase showed a c(2 x 4) symmetry, in which four Ru atoms without any oxygen atoms in between were supposed to be imaged as a bright spot. In the c(2 x 4)-2O phase, a domain boundary was identified. In the (2 x 1)p2mg-2O phase, the STM images arrays showed zigzag structures in the [1 (2) over bar 10] direction which is in accordance with the oxygen zigzag chains derived in earlier work for this phase. The UPS spectra showed that the O2p state is located at 5.9 eV in binding energy. Upon oxygen adsorption, DeltaPhi, the change in work function, increases from 0.5 eV in the c(2 x 4)-2O phase to 0.7 eV in the (2 x 1)p2mg-2O phase with respect to the clean Ru(10 (1) over bar0) surface.