What is the historical reasons to start Belgians registrations with "OO-"? For many countries, the registration number begins with a obvious letter (F for France, D for German (Deutschland I guess), C for Canada,...). For other, historical reasons have already been covered here (e.g. Where did the US 'N' registration letter come from?).

But for Belgium, I find it weird to begin with OO. Is there an historical reason for that? if so, what is it?
Since nearly all aircraft have radios, the simplest solution was to make aircraft tail numbers the same as their radio callsigns. Belgium was allocated radio callsigns with the prefixes ON through OT by the ITU, though for whatever reason, they use only prefix OO for aircraft.