Sir,
If one employee is getting his wages exactly Rs.15000/ per month, he will be coverable under ESI Act, 1948 and contribution will be payable. If his wages exceeds Rs.15000/- i.e. say Rs. 15001/- per month then he will not be coverable in said Act. If his wages exceeds to Rs. 15000/- during any month except in base months of April & October then also he will be coverable and contribution payable upto close of respective contribution period.
So far as Overtime is concerned, if the employee is coverable as per example mentioned above, then contribution on OT will also be payable regardless whether by adding the said amount of OT his wages exceeds to Rs. 15000/-.during any month.