A Number is Alien number. The A number you received from your OPT EAD is not your official A number.
If you I-140 is recently approved, please check your I-140 approval notice. You will find an A # from there. If not, just fill out "none" in your I-485 or other application forms. It really doesn't matter. If the USCIS finds that you actually have an A# in the system, they will issue the same A# for your I-485 receipt.