(III) Presents in -εινω, -ειρω, -ι_νω, -ι_ρω, -υ_νω, and -ι_ρω are formed from stems in εν, ερ, ιν, ιρ, υν, υρ with yόε- added. Here y disappears and the vowel preceding ν or ρ is lengthened by compensation (ε to ει; ι to ι_; υ to υ_). See cross37 a, cross111.
a. ὀφείλω (ὀφελ-) owe, am obliged is formed like τείνω, φθείρω in order to distinguish it from ὀφέλλω (ὀφελ-) increase formed regularly. Hom. has usually Aeolic ὀφέλλω in the sense of ὀφείλω. δείρω
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].