Is the or in doctor a postposition?

The or in doctor is not a postfix, it's a noun form.

doctor

English [?d?kt?(r)] American [?dɑ:kt?(r)]

n.

dr.; doctor, doctor of medicine; theologian; medical equipment

vt. & vi.

medicine, the practice of medicine

vt.

reparation, assembly ; to counterfeit; adulterate; to modify, embellish