ἀτεχνῶς γάρ, ὃ λέγεις, ξεναγουμένῳ τινὶ καὶ οὐκ ἐπιχωρίῳ ἔοικας:
You absolutely resemble someone on a guest tour and not a native. What is ὃ λέγεις,? with respect to what you say? acc of relation?
Phaedrus’ ξεναγουμένῳ refers back playfully to Socrates’ ἄριστά σοι ἐξενάγηται, and ὃ λέγεις simply signposts his picking-up of the word. I don’t know that the ὅ needs to be accorded a particular label, but I suppose you could always call it an accusative of respect.