In fact, Targum Jonathan on this verse indicates that the eres was displayed in the treasure house of Rabbath Ammon.
(24) He was deified and given the role of a protector of tombs, and possibly functioned as the local deity of Rabbath Ammon. The huge eres displayed there could be understood as a ceremonial bed, a cultic object indicating his presence.
When Moses says that to this day Og's eres is on display in Rabbath Ammon, what is meant is that Og is still being worshipped there.