Under U.S. law
Under U.S. law, as long as the F/O is typed in the aircraft, he can LOG the time he is actually at the controls, flying the aircraft, as PIC. I interpret this to mean that, if it's the co-pilot's leg to fly, he/she can log that time as PIC, as long as he/she is typed in the aircraft (legally able to act as pilot-in-command).
With most other countries, both pilots must be typed in the aircraft. In the U.S. this is not true.
There are airlines in the U.S. where guys are still flying as first officer...after 20 years with the company. They may have total flight time well over 15,000 hours...in some cases, pushing 20,000 hours...total flight time, and they are still first officers.
PantLoad