Many manufacturers use basically "generic" or "off the shelf" components which are THE SAME as what is sold as aftermarket parts - parts made by Dana, or TRW, or some other company - too their spec - and sold to the "aftermarket" off the same line.
In Japan you can likely buy "nippondenso" plug wires from the aftermarket as well. Here in North America you generally can not - and even some of the "dealer supplied" replacement parts are not, technically, OEM - as they are from a different supplier than the parts originally installed at the factory (and generally "locally sourced")