During the opening of the series, if you go frame by frame right when Gene is firing his gun, you'll notice the word "Bang!" appears very briefly in the middle of the gunblast.
Outlaw Star is based on a 1997 manga series of the same name by Takehiko Ito. That series is, in turn, a spin-off of an earlier parody manga series, also by Ito, called Space Hero Tales ("Uchuu Eiyuu Monogatari," also known in English as "Future-Retro Hero Story"). The Outlaw Star anime series has its own spin-off TV series, Angel Links.
Fred Luo, the homosexual weapons dealer within Outlaw Star, makes a guest appearance within the first episode of the spin-off Anime, Angel Links.