It doesn't exist. Overtime is voluntary on both sides and at an agreed rate - you either do it or you don't.
Can't speak for anyone else but in my job certain hours/duties are paid for at a preferential rate - basically by agreeing to work those hours all the time, you get a higher rate. In my case that simply means I agree to forsake my day off, go to work instead, and I get paid at a higher overtime rate.
I can also add that unlike years ago, overtime is also at a capped rate - the more you do you USED to get a bit more, but not now.