In reality the O-ring will be behind the retaining plate, between the spacer and the bearing.It looks to me as if it is to stop oil migrating along the shaft below the spacer.To save yourself a few quid, once the O-ring is in your hand, take it to your nearest bearing/Seal/O-ring specialist and get him to measure it and supply one.You could do the same with the oil seal and not pay Suzuki dealers exorbitant prices.