It depends on where you work.
Here in metro Phoenix, CVS only gives something like a $6.00 differential for anything over 42 hrs/wk.
Walgreens pays time and a half, but it's not like you can just tack a couple hours onto the end of each shift and pick up 10 hrs/wk. ...they'll give OT if you're working additional whole shifts, ie- covering for someone else.
Mail-order may be the easiest place to pick-up scattered OT, if you are able to tolerate the Rx production assembly-line environment.