The main difference between OEM and retail is the cost! OEM is intended for system builders, retail for home users. Sometimes with the OEM you will not get the box, just the sealed drive with a small installation leaflet. The retail will generally come with a nice box and a boot floppy if required also the IDE cable.
XP doesn't require the boot floppy, if your using 98se you can download the floppy from the manufacturers site. and you will probably have an IDE cable already.
Both my drives are OEM and where straightforward to install/partition/format. Buy OEM and save yourself some money. j.