The Antlers accounted for Chinese Super League club Shanghai Shenhua 2-0 in Group G and it was the 18-year old who sealed the deal a goal in the 80th minute.
Osako buried a left-foot shot from just inside the area after setting up the Antlers first goal with a neat pass to Takuya Nozawa in the first half to help the Japanese club to an important win.
Watched by national coach Takeshi Okada, Osako said he was happy but would not get carried away with his achievements.
"I'll be honest. I'm thrilled," the Japanese youth international said.
"I was hoping that I would have something to show for, and it turned out that way. But my game still needs work, and I hope I can continue to make the most of the opportunities that I get."