Overtime can be ordered by your employer only exceptionally and only for serious operational reasons, it must not become the rule. Additionally - overtime work exceeds eight hours per week (and 150 hours per year). And for overtime you are entitled to an average salary + 25% surcharge (unless you agree to take compensatory leave). If you do not accept the overtime order (according to the conditions of the previous text), your employer may "fire" you. If the ordered overtime does not meet all of the above, you can defend yourself by referring to the Labor Code, or report these practices to the State Labor Inspection Authority (www.suip.cz). But you have to reckon with the risk that the employer will want you in such cases by any means. ;-)