Elision does not occur in
a. Monosyllables, except such as end in ε (τέ, δέ, γέ).
b. The conjunction ὅτι
c. The prepositions πρό
d. The dative singular ending ι of the third declension, and in σι, the ending of the dative plural.
e. Words with final υ.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].