Original Equipment Manufacturer.
eg. Magpul "Green Label" Pmags aren't made by Magpul PTS. They basically ask some factory to make those mags for them and resell under the Magpul Green Label brand. In this case STAR is the OEM for the Green Label Pmags. Same goes for CYMA and Aftermath/Echo1. CYMA is the OEM and Aftermath and Echo1 rebrand those guns under their names usually these products go through another QC check but can sometimes just be sent to market after they're recieved at the respective warehouses/distribution channels.
Another example from computers are Power Supplies. The main OEM's include Delta Electonics and Channel Well Tech (CWT). Antec doesn't build it's own PSU's they get companies like CWT and Delta (sometimes Seasonic) to make them for them to a specified standard and just slap the Antec brandname/logo onto the PSU's. Same goes for a lot of DVD-RW's, I know for sure that Liteon makes a lot of drives OEM (or at least the drive controller or w/e) for other manufacturers and this is evident because you can crossflash firmware onto different drives (eg. A lot of the ASUS DVD-RW's crossflashable with Liteon firmware).