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Legally there is nothing wrong with doing that. You can work your overtime hours in any department within the company. You are not in any way restricted to work your hours in the department for which you would normally work. However, as overtime is still something which needs to be approved by the employer they could potentially veto it and deny you the opportunity to work across departments. This is really a decision for them. If they are happy for you to do it, then by all means do it. It is perfectly legal for you to do so. However, if they are unhappy with this, then unless your contract specifically allows it, they are able to exercise their discretion and not allow this to happen. They do have the power to do so if they really wanted to.
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