Omicron Delta region was a region of space.
In 2267, the USS Enterprise discovered the Shore Leave Planet in this region of space. (TOS: "Shore Leave") The starship returned to the region for some rest and relaxation in 2269. (TAS: "Once Upon a Planet")
Presumably, Omicron Delta was located in the Alpha Quadrant.
of "Once Upon a Planet" (as published in Star Trek Log 3) referred to this area as the "Omicron region," omitting the word "Delta" from its name.