Oberon, also designated Uranus IV, is the outermost and second-largest major moon of the planet Uranus. It is the second-most massive of the Uranian moons, and the tenth-most massive moon in the Solar System. Discovered by William Herschel in 1787, Oberon is named after the mythical king of the fairies who appears as a character in Shakespeare's A Midsummer Night's Dream. Its orbit lies partially outside Uranus's magnetosphere.

A planetary-mass moon is a planetary-mass object that is also a natural satellite. They are large and ellipsoidal in shape. Moons may be in hydrostatic equilibrium due to tidal or radiogenic heating, in some cases forming a subsurface ocean. Two moons in the Solar System are larger than the planet Mercury : Ganymede and Titan, and seven are larger and more massive than the dwarf planets Pluto and Eris.