Nouns beginning with α, ε, ο lengthen these vowels (α and ε to η, ο to ω) unless they are long by position. στρατ-ηγός
a. Some compounds of ἄγω
b. By analogy to the compound the simple form sometimes assumes a long vowel: ἠνεμόεσσα
c. Lengthening rarely occurs when a preposition or πᾶς precedes: συν-ωμοσία_
d. The lengthening in 887 is properly the result of early contraction (στρατο αγος). On the pattern of such contracted forms irrational lengthening occurs when the first part of the compound ends in a consonant, as δυς-ηλεγής (for δυς-αλεγής)
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].