The interaction of atomic oxygen with a Ru(001) surface was studied using x-ray photoelectron spectroscopy, thermal desorption, and electron diffraction. The onset of the reaction of dissolved oxygen with the Ru host starts only for equivalent oxygen coverages higher than 50 monolayers (ML). A new Ru 3d core level shifted by 0.7 eV toward higher binding energy grows with increasing atomic oxygen exposure. Further weak features 1.5 and 2.6 eV above the metallic Ru 3d binding energy appear for higher oxygen coverages. The O 1s peak shows additional components at higher binding energy than that of chemisorbed oxygen. At the highest coverage studied (880 ML), almost all the Ru metal within the probing depth is oxidized.