OEM just stands for Original Equipment Manufacturer. They are the original parts that came with the car. Genuine Nissan parts are reffered to as OEM parts because they are the exact part that came on the car originally. Unfortunately Firestone's are he oem tires for our SE's.
2001 Sentra SE w/PP auto (radium)
1998 SE-R 5spd
For computer parts, the term OEM, Original Equipment Manufacturer, means that it was not meant for retail sale. It was meant to be put into a computer system and sold as a package, not by itself. Which is why OEM products don't come in a box and usually only come along with the bare minimum software (i.e. drivers) As for the warranty issue, OEM products usually have a smaller warranty period compared to the Retail Version, and usually you don't get technical support from the product manufacturer.
2k Sentra SE Manual w/PP - Black