Most sectors have their minimum overtime rates regulated by the respective Wage Regulation Order (WRO) that regulates their activity of work. 
An  employee who chooses not to give his/her consent to work over an average of 48 hours a week cannot be asked to work beyond such an average. On the other hand, the employer can oblige an employee to work overtime (1) when the total hours of work do not exceed an average of 48 hours a week, and (2) when an employee has consented in writing to work over such an average.

Overtime Pay
An employer can offer the employee a wage that is higher than the minimum wage provided by law to compensate for overtime hours worked. This can be possible if it is clearly stated in the contract of service and circumstances actually show that the difference in the wage does truly compensate the extra hours worked according to the relevant overtime rates. However, it is preferable if  a basic weekly wage is agreed for the normal 40 hours of work while a separate amount in lieu of overtime not exceeding X hours is shown.