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.
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")
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)