text: It was held by Oliver Cromwell ( December 1653 â `` September 1658 ) and his son and designated successor Richard Cromwell ( September 1658 â `` May 1659 ) during what is now known as The Protectorate .
simplified: It was held by Oliver Cromwell ( December 1653 -- September 1658 ) and his son Richard Cromwell ( September 1658 -- May 1659 ) during what is now known as the Protectorate .