The line comes from the radio command system in Counter-Strike, Valve's tactical shooter series first released in 1999, where teammates communicate with preset voice messages instead of open chat. Ok let's go became one of the most heard commands because it is used at round start, alongside lines like bomb has been planted and enemy spotted. Clips of the commands spread through early gaming forums, frag movies and later short form video, and the short, serious delivery now works as a nostalgic shorthand for starting any coordinated push.