What is OEM pendrive and MicroSD?

To make things clear OEM means original equipment manufacturer . An OEM pen drive is usually sold by the company that directly makes the pen drives but under their own name with custom logos according to the customer's needs . A branded pen drive is a product that is not sold directly by the manufacturer but by some other company under their own brand . SD card is usually a memory card that is also a form of flash storage like the pen drive that too can be used to transfer data like the pen drive .

I think your talking about Windows Installer, an OEM pen drive is a is a bootable windows installer, rather than the CD/DVD boot. Pen drive and external boots/install faster than CD/DVD.