Complex sentence: Succeeding Gaddes as OTSL General Director was Charles MacKay, who held the post from 1985 to 2008, but he had served as OTSL Executive Director beginning in 1984.
Split into simpler sentences: Succeeding Gaddes as OTSL General Director was Charles MacKay, who held the post from 1985 to 2008. MacKay had previously served as OTSL Executive Director, beginning in 1984.