Christopher M. O'Connor, the president of Massachusetts-based Caritas St. Elizabeth's Medical Center, is leaving the hospital immediately to pursue other opportunities, according to a spokeswoman for the six-hospital chain. O'Connor, who was head of the flagship hospital of Caritas Christi Health Care for 2 1/2 years, will continue in a limited consulting role at the institution. John J. Holiver, president of Caritas Good Samaritan Medical Center in Brockton, will take the helm of St. Elizabeth's