A densely packed 1×1-O overlayer with an oxygen coverage of 1.0 (relative to the number of substrate surface atoms) was prepared on Ru (0001) by sequential dissociation of O2 and NO2 and studied by means of high-resolution electron-energy-loss spectroscopy. Two dipole-active modes were observed at 81 and 7.3meV due to a v⊥(Ru-O) stretch mode and a new surface mode, respectively. The value of v⊥(Ru-O) is shifted to higher energy compared to that for the known 2×2-O and 2×1-O adlayers at smaller oxygen coverages. These measurements indicate that the O2 dissociation is kinetically hindered for oxygen coverages larger than 0.5.