OPC and IPC
The main difference between the OPC and IPC is that when conducting an IPC no repeats are permitted. That means that if on the first exercise the candidate did not meet the standard required then the test is terminated.
With an OPC it is done under the companies check and training standard which in many cases will allow repeats to demonstrate required standard.
So if combining both OPC and IPC one would have to terminate the whole exercise if say within the first 10 minutes the candidate flew outside tolerance on say an SID.