Because Oh's contract with the Hanshin Tigers is up, he is a free agent and not subject to the posting system. His move to the Majors comes after the KBO issued a suspension for half a season to Oh for his connections to gambling with a former teammate at a casino abroad, which is forbidden by South Korea laws for its citizens. If Oh plays in Korea in 2016, he would have to miss the first 72 games.
But he declared interest in coming to the U.S. in October, and the Cardinals have reportedly been one of the teams that have expressed serious interest from the beginning.