The displacement of the prepositional object from PP (DPO) was strictly prohibited in Old English (OE). No matter how such a prohibition is theoretically analyzed, it seems clear that OE had some sort of constraint against DPO. In this paper, I address the issue of what motivated the constraint by explaining what made DPO so difficult in OE. In particular, on the basis of the discussion about relative obliqueness among OE NP arguments, I propose that what was behind the constraint is both a high degree of obliqueness of OE prepositional arguments, which was rigidly marked and represented by the preposition as an obliqueness marker, and the representation and maintenance of relative obliqueness among OE NP arguments.