OA = Original Equipment Manufacturer (OEM) Activation Microsoft uses “OA” internally to label preinstalled OEM licenses that are activated using a BIOS/ firmware embedded key (SLIC – Software Licensing Description Table). It’s not a retail key.

HP Windows 7 Pro OA is an Original Equipment Manufacturer (OEM) version of Windows 7 Professional, pre-installed on HP computers. The "OA" stands for "Original Authorization," indicating that the software is licensed for use only on the specific HP computer it was originally installed on.