An atomic oxygen species adsorbed on Ru(0001) containing subsurface oxygen has been found and is characterized by the means of thermal description spectroscopy (TDS), ultraviolet photoemission spectroscopy, and reactive CO scattering. The surplus oxygen can be adsorbed on an oxygen-rich ruthenium crystal at room temperature when more than two monolayers of oxygen are deposited in the subsurface region $(1 \mathrm{ML}=1.58\ifmmode\times\else\texttimes\fi{}{10}^{15} {\mathrm{cm}}^{\ensuremath{-}2}).$ The maximum number of oxygen atoms occupying this state depends on the oxygen content in the subsurface region. It saturates for oxygen contents higher than about 10 ML at a level of 0.25 ML. The initial binding energy of an adsorbed oxygen atom, as derived from TDS data, is lower than 0.75 eV per atom. It continuously decreases with increasing lateral adatom density. The work function $\ensuremath{\varphi}$ varies with the increasing population of this oxygen state. For saturating oxygen coverage it reaches a maximum that can be by 1.3 eV higher than the value known for a clean metallic Ru surface. The CO oxidation was performed in a beam scattering experiment and it reveals a high reactivity of this weakly bound state. At room temperature the $\mathrm{CO}/{\mathrm{CO}}_{2}$-conversion probability reaches a value of 0.18, which is nearly by one order of magnitude higher than the one found for subsurface oxygen at much higher temperatures.