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.
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2k Sentra SE Manual w/PP - Black