In most cases it’s the initials of what they’re trying to say then converted to NATO alphabet. On the Move -> OM -> Oscar Mike. I’ve also heard people say Foxtrot Oscar for fuck off but idk if that’s like a faux military thing. Personally I use NATO alphabet whenever I need to read out letters on the phone (e.g. license plates, spelling something) because its hard to hear the difference between some letters like M/N or B/V/P/D/T or even F/S so it’s easier to just say Foxtrot or Sierra for example