Source : Webster's Revised Unabridged Dictionary (1913)

OE \[OE]\ ([=e]),
   a diphthong, employed in the Latin language, and thence in
   the English language, as the representative of the Greek
   diphthong oi. In many words in common use, e alone stands
   instead of [oe]. Classicists prefer to write the diphthong oe
   separate in Latin words.