On-off keying is most commonly used to transmit Morse code over radio frequencies (referred to as continuous wave operation), although in principle any digital encoding scheme may be used. OOK has been used in the ISM bands to transfer data between computers, for example.
OOK is not very spectrally efficient due to the abrupt changes in amplitude of the carrier wave. At low to medium signalling speeds, this can be mitigated by adjusting the rise and fall rates of the carrier's amplitude. At high speeds, more efficient modulation modes (such as frequency-shift keying) are normally used instead.
In aviation, some possibly unmanned airports have equipment that let pilots key their VHF radio a number of times in order to request an Automatic Terminal Information Service broadcast, or turn on runway lights.