 Orbiting Astronomical Observatory was a series of NASA satellites flown between 1966 and 1972 for astronomy studies.The first OAO (launched April 8, 1966) used a one-off Atlas variant, mating an Agena D to the LV-3C variant of the Atlas and encased in a Centaur-type payload shroud.The remaining three launches used actual Atlas-Centaur vehicles.