The 30-year-old Canadian striker was the 2. Bundesliga's joint top-goalscorer last season with 17 goals, having netted 16 times the previous campaign for Kickers Offenbach, but this will be his first season in the top flight.
"He is a powerful striker who is excellent in the air and this makes Olivier exactly the right kind of player coach Armin Veh wanted," said Frankfurt's director of sport Bruno Hubner.
"We are delighted that, with Occean, we have been able to give our coach another of the players he wanted."
Occean has been capped 23 times by Canada, scoring five goals, and he is the seventh new arrival this summer, joining Martin Lanig from Cologne, Stefan Aigner (1860 Munich), Kevin Trapp (Kaiserslautern), Stefano Celozzi (Stuttgart), Bastian Ocipka (Bayer Leverkusen) and Takashi Inui (Bochum).
Meanwhile, Frankfurt defender Martin Amedick has been given leave of absence to seek specialist help for symptoms of burnout.
The club announced that the former Borussia Dortmund and Kaiserslautern defender has been released from training for an indefinite period "to be able to concentrate fully on the upcoming therapy".
"We are aware of our obligations towards the health of our players and we will do all we can to support Martin Amedick," said Hubner.