Depends on what you mean by Unix.
But OS X is UNIX. www.opengroup.org/openbrand/register/brand3602.htm
But OS X is UNIX. www.opengroup.org/openbrand/register/brand3602.htm
Conformance statement for El Capitan: http://www.opengroup.org/csq/repository/RID=apple%25252FXY1%...
Today there are only "Unix-like" and UNIX certified systems left. And I bet it wouldn't be too hard to certify MS Windows with SFU installed, as a UNIX.