The OS software is the same, the difference is strictly in the licensing.
An OEM license is a "system builders" license. That copy of Windows is legally tied to the first PC you install it on and can *never* be transferred to and activated on another PC. For legal purposes, the "PC" is considered the motherboard, so you can change out or replace any parts but the motheboard and still be legally OK to use the same license.
A Retail license does not have that restriction and is legally transferable, provided when you move it to a new PC you uninstall it from the old PC.