The catalog is a compilation of published data for 644 galactic O stars of luminosity class V. The catalog is intended to be useful to O-type star observers by providing a reference list for general studies on O-type stars. The catalog includes cross identifications, equatorial and galactic coordinates, visual or photographic magnitudes, B-V, spectral types, distances to the stars, radial velocities, HII region identifications, remarks, and notes.