(Adds details about BATS and its merger with Direct Edge)
NEW YORK, July 22 (Reuters) - Exchange operator BATS Global Markets said on Tuesday its president, William O'Brien, has left the company, less than six months after BATS merged with rival Direct Edge, where O'Brien had been chief executive.
BATS CEO Joe Ratterman has reassumed the role of president, the company said, without giving any reasons for the changes. Previously, Ratterman held the dual CEO/president role from June 2007 through January 2014.
There were no other executive moves, BATS said in a statement. A company spokesman declined to comment further. O'Brien could not immediately be reached.
The merger of BATS and Direct Edge received its final regulatory approvals in late January, creating the second-largest U.S. exchange, ahead of Nasdaq OMX Group and just behind Intercontinental Exchange's New York Stock Exchange in terms of equity trading volumes.
BATS' integration of Direct Edge remains on schedule, Ratterman said in a note to traders after O'Brien's departure was announced.
BATS also runs a U.S. equity options market as well as BATS Chi-X Europe, which is the largest pan-European equities exchange by market share and value traded.
O'Brien, who has a law degree from the University of Pennsylvania, was CEO of Direct Edge from July 2007 until January 2014. He was a senior vice president at Nasdaq before that.
(Reporting by John McCrank, Editing by Franklin Paul; and Peter Galloway)