The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

OE
       Outlook Express (MS, Internet)