What Does OEM Actually Mean?
OEM stands for Original Equipment Manufacturer. An OEM part is manufactured by the same company — or to the same specifications — as the part that was installed in your car when it left the factory. When you buy an OEM part, you are buying the exact same component your car was built with, just in a box that may say the supplier's name rather than the car brand's name.
For example: the alternator in your BMW 3 Series was likely made by Bosch or Valeo — not BMW. BMW sourced it, fitted it, and it became an OEM part. You can buy the identical Bosch alternator from an independent supplier like BuyOEMOnline for significantly less than the BMW-branded equivalent, because it is the same component.
What Is an Aftermarket Part?
An aftermarket part is made by a third-party manufacturer who was not the original supplier to the car maker. These companies reverse-engineer the original part and produce their own version — sometimes to a high standard, sometimes not.
Aftermarket parts range from near-OEM quality (e.g. Brembo brake pads, Gates timing belts) to dangerously poor quality (unbranded parts sold at minimal cost online). The key variable is the manufacturer, and the problem is that without detailed knowledge it is very difficult for a consumer to tell the difference.
OES: The Third Category Most People Miss
Between OEM and aftermarket sits OES — Original Equipment Supplier. These are parts made by the original factory supplier (Bosch, Valeo, ZF, Continental, etc.) but sold in their own branded packaging rather than under the car manufacturer's name. OES parts are functionally identical to OEM parts. They are made in the same factory, to the same specifications, on the same production line.
OES parts are typically 15–35% cheaper than dealer-sourced OEM parts and represent the best value for money for quality-conscious buyers.
Quality Comparison
OEM / OES: Manufactured to the exact tolerances specified by the car maker. Every dimension, material, and performance characteristic matches the original. Tested as part of the complete vehicle system. Covered by the part manufacturer's warranty.
Quality aftermarket (Brembo, NGK, Gates, Mahle, Mann): Often excellent quality, sometimes genuinely equivalent to OEM. These brands supply many car makers and understand automotive specifications well. A reasonable choice for commoditised parts like oil filters and spark plugs from reputable brands.
Generic / unbranded aftermarket: Unpredictable quality. May fit and function initially, then fail prematurely. In safety-critical applications (brakes, steering, suspension, timing systems), this is not an acceptable risk.
When OEM Is the Right Choice
- Safety-critical components: brakes, steering rack, suspension arms, wheel bearings, timing belt/chain, airbag components
- Electrical and electronic parts: sensors, ECUs, ABS modules — aftermarket clones frequently cause diagnostic codes and intermittent failures
- Engine internals: gaskets, seals, timing components — tolerances matter enormously here
- Warranty preservation: if your car is under manufacturer warranty, fitting non-OEM parts can void coverage for related components
- Premium vehicles: BMW, Mercedes, Porsche, Audi — these cars are engineered to tight tolerances where inferior parts cause performance and reliability problems disproportionate to the saving
When Aftermarket Can Be Acceptable
- Low-criticality consumables from well-known brands (wiper blades, cabin filters from Mann or Mahle)
- Classic or older vehicles where OEM parts are discontinued
- Performance upgrades where you intentionally want to exceed OEM specification (Brembo performance brake pads, for example)
Price Reality Check
The most common argument for aftermarket parts is price. But the comparison is often made against dealer retail prices for OEM parts — which are genuinely inflated. When you buy genuine OEM parts from an independent supplier like BuyOEMOnline, the price difference versus quality aftermarket narrows significantly. For many parts, genuine OEM from an independent source costs the same or less than branded aftermarket from a motor factor.
The calculation changes further when you factor in labour: if an aftermarket part fails in 18 months and requires another workshop visit, the "saving" disappears. A genuine OEM part fitted correctly should match the original service life of the vehicle.
The Counterfeiting Problem
A growing issue in the aftermarket space is counterfeit parts sold as OEM. These are low-quality components packaged in convincing-looking boxes bearing the car maker's logo. They are illegal, dangerous, and surprisingly common online. The safest protection is buying from reputable suppliers with verified supply chains. At BuyOEMOnline, all parts are sourced directly from authorised OEM distribution — not secondary markets or grey imports.
Our Recommendation
For the cars we specialise in — BMW, Mercedes-Benz, Audi, Volkswagen, Porsche, Volvo, Toyota — we recommend genuine OEM or OES parts for all mechanical work. The price premium over well-sourced OEM (as opposed to dealer retail) is small, and the reliability benefit is real. Browse our full OEM parts catalog to see current pricing.