An Oort Cloud is a cloud of comets situated at the far reaches of some (or all) planetary systems, including Sol and Vela. It is in an Oort Cloud that the Tobermoon was attacked by an unknown enemy; Hayes also makes a humorous reference to Oort Clouds telling The Captain about the Umgah Caster: "This thing would be great for practical jokes. You could scream `Boo!' from the Oort Cloud and scare the pants, or whatever, off everyone in the system." — a hint to using the device to impersonate Dogar and Kazon.
See also: Wikipedia article on Oort cloud