DirectX is more than OpenGL.
OpenGL is purely graphical presentation (and animation, to an extent), the scope of DirectX is far far wider, including networking, sound, and a lot of other game related services.

OpenGL knowledge will be valuable no matter what platform you're going to work on. DirectX knowledge is Windows specific.

OpenGL also seems to change its specs less often than does DirectX, making it maybe a better thing to learn for future use.