Any of eight nearly identical solar observatories launched by NASA from 1962 to 1975. OSOs are designed mainly to point several ultraviolet and X-ray telescopes at the Sun from a platform mounted on a cylindrical wheel. OSO 1, active from its launch in March 1962 to August 1963, was a 200-kg spacecraft deployed to measure solar electromagnetic radiation in the ultraviolet, X-ray, and gamma-ray wavelengths, and to investigate dust particles in space. The University of Minnesota gamma-ray experiment aboard measured the intensity and direction of low-energy gamma rays in space.

OSO 5 had the longest lifespan, from January 1969 to July 1975. It studied solar X-ray bursts and measured the diffuse cosmic background. OSO 8, launched in June 1975 and active until October 1978, had four additional instruments to study other celestial X-ray sources.