Yep..some groups allow or even encourage OT. The group I worked for didn't.My original offer was at GS-7 but I was told I could negotiate starting pay so I did. Started at 45k in 2000. My group got told we were working OT and you had to have a good excuse why you weren't or weren't working a certain amount. A few years ago I worked 40 over what would have been my three day weekend. (Starting Thursday evening with only a 2 hr notice)
I ended up having to get a pay waiver, without one you can't earn more than a GS-15 step 10 in a pay check or you just don't get paid for it. An SES has to sign off on that.
About 4-5 months before I left CS, I was offered the job of Chief Scientist at the Radford Army Ammunition Plant. It was a GS-13 position and the guy that held it, and who picked me to replace him, had been the government chemical engineer assigned to help open the plant in 1942!
The CO of Radford plant, an Army Colonel, went through all sorts of hoops to get my hire approved (I'd only recently gotten to GS-9, after all, so it required a single jump from GS-9 to GS-13). I gave it a lot of thought and even went property hunting in the area around Smith Mountain Lake (I gotta be near water..even if just a lake). In the end..I turned it down and left the CS. . Boy was that Col pissed off.