The following is based on Van Buren and has not been confirmed by canon sources.
Ouroboros is a village in Western Utah. It is where Hecate, the mad tribal "goddess" resides, as well as the Vipers, a group of raiders from the West who serve her. Jordan Dae, the captain of Ouroboros' elite city guard, also resides here.
Ouroboros did not appear nor was mentioned in any of the published games, but was to appear in the canceled Fallout 3 project by Black Isle Studios codenamed Van Buren. It is not certain if it will appear in any future Fallout game.