The younger brother of Abdullah Öcalan, Osman studied at teachers' training college before joining the PKK when it was founded in 1978 and spending two years in Libya. He joined the central committee in 1986, and the executive committee in the 1990s, becoming virtually second in command of the PKK, but in 1992 he suffered disgrace after signing a truce with the two main Iraqi Kurdish parties, the KDP and the PUK, and was jailed by the PKK.