An Oort cloud is a spherical cloud located around a star or other celestial body and is composed of ice particles, dust and other debris, and is situated where the star's influence are counter-balanced by the aggregate of forces external to the star. It is named after the human astronomer Jan Hendrik Oort.
In 2369, Chief Miles O'Brien hid a dozen sensor probes in the Oort cloud of the Bryma star system, in order to detect the approach of any Maquis vessels that would attack the Cardassian colony on Bryma. (DS9 episode: "The Maquis, Part II")
Within the Oort cloud of the Narisian system, the Narisian Garamet found a B'rel-class Klingon bird-of-prey and a Starfleet shuttlecraft that originally belonged to the USS Senzig, both of which were likely abandoned there by the Borg.
After using that bird-of-prey to travel back in time, Montgomery Scott inadvertently caused an alternate timeline to form where the Borg Collective had successfully assimilated most of the Alpha Quadrant, including the Sol system, around which they erected a sensor shield which encompassed even the system's Oort cloud. (Star Trek novel: Engines of Destiny)
In an alternate January 2374, the USS Voyager sheltered behind a large ice dwarf situation in the Oort cloud of a Borg-occupied star system. From this location, they witnessed ten Species 8472 battleships destroying three Borg occupied planets. While attempting to escape the Oort cloud, Voyager was attacked by one of the battleships and suffered critical damage and lost numerous personnel. (VOY - Infinity's Prism novel: Places of Exile)