As noted above, 233 OCU was created out of the HCU in October 1970. The units first C.O. was Wing Commander L. Baker, with its main role being the conversion of new pilots on to the Harrier. The first course for new (as opposed to second-tour) aircrew began in March 1971.
The unit is the primary RAF user of the two-seat Harrier, as well as having numbers of single-seat aircraft on strength. With the introduction of the Harrier GR.5 and GR.7 Into the RAF, 233 OCU initially continued to use the first generation T.4/4A trainers, only changing to the second generation T.10 in the mid-1990s. The unit has also made extensive use of simulator training over the years.
As well as training Harrier crews for the RAF 233 OCU has also provided initial Harrier training for Royal Navy crews destined to fly the Sea Harrier as well as having trained American, Indian and Spanish Harrier pilots. From its inception the OCU has been based at Wittering. In 1992, with the reduction in the overall size of the RAF following the end of the Cold War, 233 OCU was renumbered as 20(R) Squadron, reviving the association of that unit with the Harrier, which it had flown in Germany during 1970-77.