This release of Darius Gaiden, similar to other versions marked with version numbers and dates, indicates a specific iteration in its development and distribution history. The 'O' might denote an 'other' region release, potentially a version optimized for a particular area outside of Japan and the USA, or a version with a unique set of modifications. The version number facilitates tracking specific changes over the game's lifetime.

Darius Gaiden - Silver Hawk (Ver 2.5O 1994/09/19)

Darius (World)