COD orders are order that were completed using the payment method 'COD/Pay Later' and will be paid later by the customer.
On your Sales Reports, orders that were originally COD but have been paid will show under the payment method that was used on the date payment was received.
If you want to see orders that were placed using COD and are still unpaid, check the box next to "COD/Pay Later" then Show Report and all orders still unpaid and marked COD will appear under the 'COD/Pay Later' section of report.
Orders that has been paid for initially with "COD/Pay Later" will have the text COD/Pay Later" next to their order number regardless if they have been paid for with another payment method.
Please note: The Product Category Report will only show order that have been paid, so COD orders will show on report the date they are paid.