The formula for OT per hour under the Employment Act is (salary/26x8) x 1.5, if you work for more than your "normal hours of work" . If 9am to 5pm is your normal hours of work, then any work done after 5pm is overtime. Section 60D of the Employment Act may be useful to you:
60D(3)(a) For any overtime work carried out in excess of the normal hours of work, the employee shall be paid at a rate not less than one and half times his hourly rate of pay irrespective of the basis on which his rate of pay is fixed.
(b) In this section, “overtime” means the number of hours of work carried out in excess of the normal hours of work per day:
(c ) For the purposes of this section, section 60, section 60D(3)(a) and section 60I, “normal hours of work” means the number of hours of work as agreed between an employer and an employee in the contract of service to be the usual hours of work per day and such hours of work shall not exceed the limits of hours prescribed in subsection (1)..."