Using highly resolved electron energy loss spectroscopy, we have investigated the (2 x 2)-(O+CO) coadsorbate layer on Ru(001). All adsorbate-derived modes have been determined. CO-derived modes at 56, 411, 462, and 2048 cm(-1) are found for the frustrated translation parallel to the surface, the frustrated translation perpendicular to the surface, the frustrated rotation, and the internal C-O stretch mode, respectively, at the F point. The oxygen vibrations within the (2 x 2) coadsorbate structure are found at 431 and 525 cm(-1) for the Ru-O bending and stretching vibrations, respectively. Off-specular data reveal a dispersion of the internal CO and the Ru-O stretching vibrations due to dynamical dipole-dipole interactions; the other modes are dispersionless within our detection limits. Overtone and combination modes are detected and assigned. The details of the frustrated translation parallel to the surface and its overtone are compared to data for the (root3 x root3)-CO layer and discussed with respect to a previously proposed tilting of the molecular axis in the (2 x 2)-(O+CO) layer. A proposal is made to resolve apparent disagreement between earlier quantitative low-energy electron diffraction measurements and density functional theory calculations.