OReillys= aftermarket made in mexico. Unless it's a genuine GM or AC DELCO, you got aftermarket. Put an ampmeter on the alternator output wire and see just what it is producing at idle. Like I said earlier, beings you got an aftermarket, there is also a chance you got a CS-130 D instead of an AD230. Most people do not know how to tell the difference as they are rather subtle. Theres nothing really wrong with a CS-130D, but they don't seem to put out as much at idle as the newer AD series does.Not aftermarket, no. OEM replacement from O'reillys.
I'll check the output cable also.