Why do Czech Airlines have the airline code (IATA airline designator) "OK"?
It starts with the International Telecommunications Union, which assigned radio call sign prefixes for each country in 1927. The OK prefix was assigned to Czechoslovakia. Other than the largest countries (US, USSR, UK, France, etc.) there seems to be no specific pattern to the assignments. When ICAO assigned aircraft registration prefixes for each country it usually based them on the ITU prefixes. In 1947, when ICAO assigned 2-digit airline identifiers they chose to use the country’s registration prefix for Czechoslovakia’s flag carrier. IATA originally based their airline ID’s on ICAO’s 2-letter ID’s. In 1982 ICAO changed to their current 3-letter format.