In Calamari, you can manage overtime using only the Leave module or having the Leave and Clockin modules. We will describe two scenarios below:
Having only the Leave module:
Create a new Absence Type
To be able to manage the overtime process, you need to create the Overtime Absence type. For this, go to Configuration → Absence types → Add:
In the General tab: choose hours unit
In the Accrual tab: create a rule that accrues 0 hours

Adjust Entitlement
When the employee spends more hours at work, we can assign overtime to be collected using the 'Adjust entitlement' function. Simply, go to Configuration → Employees → select the employee → Adjust entitlement:

Important: If you want to import data in bulk for many employees, use balance import from the file. Check out the details here:
How to import entitlement balances to Calamari?
Assign overtime to be collected
Select the year and absence type, then add the hours to the employee:

Claiming overtime
The employee can now collect overtime in the form of a request:

Having both the Leave and the Clockin modules
Checking the Timesheet report
If you use both modules, you can check the overtime hours in the Timesheet Report:

This report shows how much overtime the employee has recorded, the deficit, and moreover, the overall working time balance.
This is certainly a more accurate way of checking overtime.
Allocating overtime
After checking the Timesheet report, proceed in exactly the same way as described for the Leave module, i.e.: adjust the entitlement, assign overtime, and finally, claim it back.