Ok as a basis for my comments I worked for Ingram Micro (a large distributor of computer products) for a few years.
You usually get the same warranty with OEM or Retail, the manufacturer must state if there is a difference.
For instance in the case of Seagate and Quantum, IDE HDD's in OEM form had a 3 year warranty (SCSI had 5yr). OEM drives were sold to resellers, retailers, system builders etc, they then can advertise this 3 year warranty or use it in combination with their own warranty.
This means that the end user basically applies for a replacement drive to whomever they bought it off. The application works it's way back up the channel eventually reaching the manufacturer. By this stage the retailer should have supplied a replacement drive to the customer.
With HDD's there is no physical difference between OEM and Retail drive. With Retail you are paying extra for the box, cables and software (if there is any)
From my experience IBM offer excellent warranty's and servicing, you won't be sorry buying their HDD.