Originally Posted by Brian Ratliff

The question as posed makes no sense.

There is no such thing as OEM on a bike. There is a frame, then someone else (or lots of someone elses) makes everything else. A bike you buy complete is a frame a frame builder sold you with a bunch of name brand parts stuck on it that the frame builder was able to get cheap because he buys in quantity. There are no "OEM vs. aftermarket" or anything like that like you find in the automobile world. This is not the automobile model for manufacturing. This is the "custom build" (for lack of a better word) model with every manufacturer specializing on one thing, the frame builder acting as arbitrator for buying components in quantity, and the bike shop acting as the assembler.

Another way to put it is the automobile model is fully integrated into a vertical supply structure with every part owned and manufactured by the auto maker (or if not strictly manufactured by the auto maker, then "OEM-ed", meaning someone else manufactures the part under contract and the auto maker puts its sticker on it). This is as opposed to the custom build model, which is fully distributed with different companies building different parts under their own name.

It works this way (I should add that this is for road bikes; dept store bikes are more the OEM model) because each and every bike is custom. There is no such thing as a "stock" bike. Every part is adjustable and replaceable and everyone makes their adjustments and replacements over time.