ESI will be payable on O.T. wages irrespective of coverage. If you are paying overtime to an exempted employee, then you still have to deduct ESI, but that deduction should be based on the overtime amount only. Please go through
esic.nic.in and see the O.T. definition there.
OVERTIME ALLOWANCE
In the case where the employer, upon finding the need to expedite work in addition to normal working hours, offers the employee the opportunity to work overtime, a concluded implied contract emerges between the employer and employee when the employee accepts the overtime work. Both the remuneration received during regular working hours and for overtime constitute a composite wage, falling within the definition of Sec.2(22) of the ESI Act. Consequently, contributions are payable on overtime allowances. However, overtime allowances are considered wages solely for contribution purposes and do not affect employee coverage under the Scheme.
This perspective was upheld by the Supreme Court in its judgment on 6.11.96 in the case of Indian Drugs & Pharmaceuticals Ltd. Vs. ESIC, in Civil Appeal No.2777 of 1980. (Old instructions issued via memo No.3-1(2)/3(1)/68 dated 31)