[1] Belgium was not considered a major operator of aircraft and was allocated the prefix and first letter suffix O-B.
[1] At the 1927 International Radio-Telegraph Conference, new radio callsigns were allocated to Belgium, comprising ON, OO, OP, OQ, OR, OS and OT.
[1] In 1928 the International Convention of Air Navigation re-allocated the aircraft registration prefix to align with these radio callsigns.
[6] An Aero Commander 560F was operated for royal flights between 1961 and 1973 without a serial number but display its radio callsign OT-CWB instead.
[6] The Belgian Navy have operated a number of shipborne helicopters which were allocated serials with single letter prefix although the radio callsigns were also painted on the aircraft in a similar format to registrations.