The Oneamisu sector was a region of space that contained the Braslota system.
In 2365, the USS Enterprise and the USS Hathaway conducted a Starfleet battle simulation in the Braslota system. (TNG: "Peak Performance")
Presumably, this sector lay within Federation borders, but the presence of the Ferengi marauder Kreechta indicated it could have been open territory.
Most of the simulation information consisted of in-joke references to Anime (Japanese animation), including The Wings of Honneamise
No Tsubasa) for this specific example.