O.E.M. stands for Original Equipment Manufacturer.
This means that the part produced by the brand’s name is the same as the original brand.
For example: TONAR MMC Cartridges are OEM as they are now produced with the same machines with exactly the same specifications as the original B&O cartridges.
So the only difference is the name of the brand, not the item itself.
In technical way they are exactly the same products!
If the mentioned example the TONAR ones are better in quality for the reason that they are produced in this period while the B&O cartridges were produced decades ago so a “new” B&O is unused but stored for a long while somewhere. This does not mean these are not good, but if they are not stored properly (moist/heat/sunlight etc.) they might have an issue or get an issue sooner so that’s something to keep in mind when choosing between the older (often cheaper because of the aging) original and a newer OEM item.
