Oliver —, Lord Protector of the Commonwealth from 1653-58 (8)
I believe the answer is:
cromwell
(Other definitions for cromwell that I've seen before include "Roundhead leader" , "He gave the Irish the choice of going ''to hell or to Connacht''" , "Oliver, Puritan leader during 17th century English Civil War" , "Lord Protector (sixteen fifty three to sixteen fifty eight)" , "Oliver --, Lord Protector" .)