YU-NO
Genres: Sci-Fi, Hentai
Type: OVA, Episodes: 4
Aired: Oct 23, 1998 to Sep 24, 1999
Studio: PP Project
Source: Visual novel
Rating: Rx - Hentai
Synopsis:
Takuya's father died during an accident while he was doing research. Worried about his young stepmother, Ayumi, Takuya follows her to Ken point. Watching her and her best friend, Mitsuki, from behind a rock, a sudden flash of light turns into an unearthly beautiful woman. The mysterious woman slips a ring on his finger, and Takuya's travels into a parallel world begins. 

This ring is the key between Takuya and his different realities.

(Source: AniDB)