Conversion units and operational conversion units (OCUs) were training units of the Royal Air Force (RAF).

This article includes a list of general references, but it lacks sufficient corresponding inline citations. (June 2020)

With the introduction of new heavy bombers, the four-engined Short Stirling, Avro Lancaster, and Handley Page Halifax, the Royal Air Force introduced heavy conversion units (HCU). These HCUs began forming in late 1941, to qualify crews trained on medium bombers to operate the heavy bombers before final posting to the operational squadrons. Some of the HCUs were involved in bombing operations over Germany.

After the end of the Second World War, the role of the HCUs was taken over by the operational conversion units (OCUs). Although the units had nominal bases, different flights and individual aircraft usually were detached nearer the operational bases.

RAF OCUs are training units that prepare aircrew for operations on a particular type or types of aircraft or roles. Some OCUs have a shadow, or reserve, squadron designation, which is used if the unit has a war role.

F-35 Lightning – RAF Marham

• 207 Squadron^[1]

Typhoon – RAF Coningsby

• 29 Squadron^[2]

Hawk T.2 – RAF Valley

• 4 Squadron^[3]

Support helicopter (Puma and Chinook) – RAF Benson

• 28 Squadron^[4]

Air Mobility (Atlas and C-17) – RAF Brize Norton

• 24 Squadron^[5]

ISR (Wedgetail, Poseidon) – RAF Lossiemouth

• 42 Squadron^[6]

ISR (Reaper, Protector,Rivet Joint and Shadow) – RAF Waddington

• 54 Squadron^[7]

Tutor – RAF Wittering

• 115 Squadron^[8]

Some aircraft types operated by a single squadron, which includes most transport aircraft, and most ISTAR aircraft, are not big enough to need a dedicated OCU squadron for their training requirements; they may only have a few at any time. Therefore, smaller squadrons also incorporate training facilities to allow them to process aircrew onto their aircraft type. Some roles, such as ISTAR, require more rear crew for the aircraft, such as weapon systems officers (WSOs) and weapon systems operators (WSOps) – their training can be more generalised, and this is carried out by 54 Squadron at RAF Waddington. This reduces the training requirement on the individual ISTAR squadrons, by providing aircrew who require only aircraft conversion training when they reach their squadrons.

Pilots are trained directly onto their aircraft type by the frontline squadrons using training "flights". An example of this is the OCU flight of 24 Squadron, which trains new Atlas and C-17 aircrew onto the aircraft.

115 Squadron trains flying instructors for all three branches of the British Armed Forces. 207 Squadron trains both RAF and Royal Navy pilots on the F-35 Lightning.

OCUs are monitored by the RAF's Central Flying School to maintain training standards. Training is delivered by qualified flying instructors (QFIs) and qualified weapons instructors, and frontline squadrons also have qualified individuals to deliver continual and refresher training after the OCU. OCUs generally also provide training for those aircrew selected to become QFIs on an aircraft type – an example would be the now disbanded 208(R) Squadron, which previously trained aircrew for the now disbanded 100 Squadron, QFIs for 208(R), and provided refresher training for aircrew joining the RAF aerobatic team, the Red Arrows, on the Hawk T.1.