Λοξίᾳ δὲ μέμφομαι,
ὅστις μ᾽ ἐπάρας ἔργον ἀνοσιώτατον,
τοῖς μὲν λόγοις ηὔφρανε, τοῖς δ᾽ ἔργοισιν οὔ.
Is there any particular reason for using ὅστις here?
also, it this double acc with ἐπάρας or rather a direct object and an acc of respect?
You can find either ὅς or ὅστις with a relative clause of cause. Sometimes the two pronouns are functionally equivalent, especially later but occasionally already in the 5th century. (For more, I recommend Philomen Probert’s book on relative clauses). The double acc is unusual. You would expect an inf. There are several conjectures (e.g. τελεῖν, δρᾶσαι), and Diggle obelizes ἔργον ἀνοσιώτατον.