Oxygen overlayers with different coverages on Ru(0001) were prepared by dissociative adsorption of O2, and sequential dissociation of O2 and NO2. The high-resolution electron energy loss spectroscopy measurements showed a strong coverage dependence of O-Ru strectch mode. The vibrational energy of this mode shifts from 54 meV for the lower coverage limit (θO→0.0) upward to 81 meV for densely packed (1×1)-O overlayer (θO=1.0).