In 2011 he ran a 5000 metres track best of 13:31.27 minutes and came second over 10,000 metres at the Hyogo Relays. Osako was chosen to represent his country at the 2011 Summer Universiade, where he defeated Stephen Mokoka in the 10,000 m to take the gold medal, becoming the first Japanese since Yasuyuki Watanabe to win that title. At the start of the 2012 season he was the top Japanese finisher at the Chiba Cross Country (fourth overall) and had a narrow win at the Fukuoka Cross Country, beating Yuki Sato. Osako improved his track bests in 2012, running 3:42.68 minutes for the 1500 metres, 7:54.68 minutes for the 3000 metres and 27:56.94 minutes for the 10,000 m.