Osako was born in Minamisatsuma on May 18, 1990. After graduating from Kagoshima Josei High School, he joined J1 League club Kashima Antlers in 2009. He played many matches from first season and Kashima won the J1 League champions in 2009. In 2013, he scored 19 goals and was selected Best Eleven award. In January 2014, he moved to German 2. Bundesliga club 1860 Munich. In summer 2014, he moved to Bundesliga club Köln. He played many matches for the club in 4 seasons. However Köln was relegated to 2. Bundesliga end of the 2017/18 season. In 2018, he moved to Werder Bremen.