O'Pake died of complications from heart-bypass surgery at Reading Hospital and Medical Center, where he had remained since the Nov. 22 operation, according to longtime aide Jim Hertzler.
A Democrat from Berks County, O'Pake was first elected to the state House of Representatives in 1968, and to the state Senate four years later. He died midway through his 10th consecutive Senate term.
As the first chairman of the Senate Aging and Youth Committee, O'Pake helped write the child protective services law that helps child abuse victims and was a leading advocate of the state's pioneering Children's Health Insurance Program, which provide health insurance to uninsured children and teens who do not qualify for Medicaid. He also fought for legislation that created the state Department of Aging and separate offices of aging in every county.
Last month, fellow Senate Democrats re-elected O'Pake as minority whip, the No. 2 caucus leadership position.
"Mike was a friend and colleague who could be counted on for kind words and sage advice in the most trying circumstances," said Minority Leader Jay Costa, D-Allegheny. "He worked across the aisle and built bridges trying to find solutions that help all Pennsylvanians."
Funeral arrangements were incomplete Monday.