Quite oft. ὅστις takes the place of the simple rel...; this occurs occasionally in ancient Gk. usage..., but more freq. in later Gk..., esp. in Luke's writings (BDAG s.v. ὅστις 3).
The neuter nominative/accusative singular ὅ τι is written with a space to disambiguate it from the conjunction ὅτι. In Modern Greek, it is written ὅ,τι, with a hypodiastole (a precursor to the comma which was used for word separation, as necessary, prior to the invention of spaces).
The prepositional phrases ἀφ’ ὅτου..., ἕως ὅτου..., and μέχρις ὅτου are fixed expressions (BDAG s.v. ὅστις 6). ἕως ὅτου occurs in Mt 5:21; Lk 12:50, 13:8, 15:8, 22:16, 22:18; Jn 9:18; and the LXX. μέχρι ὅτου occurs in Tobit 5:7.