The vocative is the same as the nominative:
a. In stems ending in a stop ( cross16) consonant (except those in ιτ, ιδ, ι_θ; ντ in nouns): ὦ φύλαξ
b. In oxytone stems ending in a liquid and not taking ς to form their nominative ( cross242): ὦ ποιμήν
c. In all participles.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].