As we all know, OEM headphones have a same sound quality as the original ones, and the price of OEM headphones range are usually far below the Original one.

OEM just mean the headphones are from the company that originally made them anyways. Original Headphones would be bought through main company that asked the 3rd party (OEM) to make them for them.

For example, if a part on your Sennheiser is messed up, you can go to Sennheiser and get it or you can go to the Original Equipment Manufacturer and get it straight from them...cut out the middle man.

OEM's are products, such as earphones, made by a certain company and then bought by another and given the latter company's brand name. I don't think OEM isn't another fancy term for fake since the OEM products are bought by a certain company. The term fake would better be applied to products that are made to "look" like the original thing without the company's legal consent, knowledge, etc.

I would personally consider OEM's to be of good quality, perhaps even at par with (if not better than) the original. Additionally, a good company such as Sennheiser would not risk its name and credibility in purchasing OEM products of poor quality.