We are not allowed to work any overtime because that will change the total hours available in the contract. Though occasionally we are here late some days - we will be allowed to go early the next day if that happens. Never any money gets paid for that and we are not allowed to accumulate the time off either.
I guess, with the exception of the accumulation, it's pretty much the same story here. Occasionally I get asked to do work outside of normal hours (eg. weekends), and get paid overtime for it, but that's rare - happened like 3 or 4 times in total.