The Oort cloud (sometimes called the Öpik-Oort cloud) is a spherical cloud around a star or other celestial body, composed of tiny ice particles, dust and other debris, situated where the star's influences are counter-balanced by the aggregate of forces external to the star. In 2369, to detect Maquis ships coming into the Bryma system to launch a possible attack on the Cardassian colony on Bryma, Chief O'Brien hid a dozen sensor probes in the Oort cloud of the star system, where the Maquis wouldn't be able to detect them. (DS9: "The Maquis, Part II").

Our own Solar System's Oort cloud is a spherical cloud of comets believed to lie roughly 50 000 AU, or nearly a light-year, from the Sun. The distance places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The Kuiper belt and scattered disc, the other two known reservoirs of trans-Neptunian objects, are less than one thousandth the Oort cloud's distance. The outer extent of the Oort cloud defines the gravitational boundary of our Solar System.

The Oort cloud is thought to comprise two separate regions: a spherical outer Oort cloud and a disc-shaped inner Oort cloud, or Hills cloud. Objects in the Oort cloud are largely composed of ices, such as water, ammonia, and methane. Astronomers believe that the matter comprising the Oort cloud formed closer to the Sun and was scattered far out into space by the gravitational effects of the giant planets early in the Solar System's evolution.

Although no confirmed direct observations of the Oort cloud have been made, astronomers believe that it is the source of all long-period and Halley-type comets entering the inner Solar System and many of the Centaurs and Jupiter-family comets as well. The outer Oort cloud is only loosely bound to the Solar System, and thus is easily affected by the gravitational pull both of passing stars and of the Milky Way galaxy itself. These forces occasionally dislodge comets from their orbits within the cloud and send them towards the inner Solar System.