In the nominative ὅς, ἥ, are usually thus written. Some write ὅ, ἥ, οἵ, αἵ when these words are used as demonstratives; but ὃ μέν . . . ὃ δέ is rare.
a. The forms ὅς, ἥ, here apparently relatives with an older demonstrative force, may be in reality demonstratives, ὅς being the demonstrative (article) ὁ to which the nominative sign -ς has been added. From this ὅς may be derived, by analogy, the demonstrative use of ὅ, and of οἶς, οὕς in fixed expressions ( cross1110).
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].