OE vs. OEM
OE (Original Equipment or Genuine BMW)
These are the parts that you get EXACTLY as if you are walking up to an authorized dealership and buying a part at the parts counter. (Example: Oxygen Sensor, BMW Part # 11 78 1 735 499). These parts usually come in BMW-marked packaging.
OEM (Original Equipment Manufacturer)
OEM parts are parts that come from the same factory that makes your Original BMW parts, but they are boxed in their respective brands. BMW actually makes very few components themselves. Nearly the entire car consists of components and sub-assemblies sourced from various manufacturers or Tier-1 suppliers. (Example: Oxygen Sensor, Bosch Part #13231, which cross references to: BMW Part # 11 78 1 735 499). These parts are sold directly from the manufacturer to distributors, suppliers, or retailers and not through the BMW dealer network. This usually means savings for the end consumer while still having the same OE standards that the vehicle manufacturer specified. They are identical - a lot of times you can find where the OEM part has the BMW logo removed! In order to be labeled as an OEM part, it must meet the exact same design as the original part, regardless of where in the world it was manufactured. Any deviation from the original BMW spec is not considered OEM. In addition, in Germany for a part to be labeled "OEM" it must achieve German TUV certification.
Some OEMs may choose to produce a performance version of the original design. Technically speaking, these are no longer OEM parts but since they are coming from the original supplier you can be sure the quality is excellent and on par with the original. We generally refer to these as 'Performance Alternatives'.
Aftermarket parts are produced by manufacturers that have no connection with OE. These are generally copied from OE parts but produced and sold for less cost than an OE/OEM. They are not necessarily inferior, unsafe, or poorly made. But since they have not achieved OEM or TUV status their quality is not guaranteed. In some cases, an aftermarket part may be superior to OEM but it is rare. Bottom line: ask your supplier for their opinion. But if they can't give you an honest answer then look elsewhere. Ask which brand has the highest return rate. If you show a little interest and trust his/her opinion you should get what you want. It's not all about the price.
"The bitterness of poor quality remains long after low pricing is forgotten," - Leon Cautillo, author.