What are the "OEM" keys in the System.Windows.Forms.Keys enumeration?

They were introduced with the Windows API, they are not specific to WPF.

First, these are KEY codes, not character codes: they identify a key on your keyboard, not a character - e.g. the 'A' character is usually generated by pressing the A key while holding down a SHIFT key or having CAPS LOCK active.

The OEM keys are the keys that vary with local keyboards. Where the US keyboard has brackets and braces, german keyboards have umlauts.

They are called "OEM" because the Original Equipment Manufacturer (of the keyboard) was responsible for defining their functionality.