Here I am attaching the sheet, which may help you in calculation of Overtime.
In the first sheet, you have to just put the In and Out time of the employee. Based on the working hours, you can list the overtime or the lacking of time in the mentioned collumn. The Over-time will be calculated which is more than 8:00 hrs.
While the another sheet helps in calculating the monthly Over-Time hours of the employees.
Just go through the attachment !!!