You only have to work overtime if your contract says so.
Even if it does, by law, you cannot usually be forced to work more than an average of 48 hours per week. You can agree to work longer - but this agreement must be in writing and signed by you.
Unless your contract guarantees you overtime, your employer can stop you from working it.
However, your employer cannot discriminate against anyone, for example by stopping some employees from working overtime while letting others do so.