Overtime is to be paid separately, with a surcharge of 50% on the hourly wage. You can find possible exceptions in your collective agreement.
In the event of compensatory time, you are also entitled to the surcharge of 50%. That is, 4 hours of overtime = 6 hours of compensatory time.
Whether overtime is to be paid out or reduced by compensatory time is a matter of agreement and should be regulated in the employment contract.