Osako broke Colombia's resistance in the 79th minute after Juan Quintero's free-kick cancelled out an early Shinji Kagawa penalty at the Mordovia Arena, reports Xinhua news agendy.
The South Americans were forced to play almost the entire match with 10 men after Carlos Sanchez was sent off for a deliberate handball in the area.
The result leaves Group H wide open given that 61st-ranked Japan were the least fancied of the four teams in the group, which also includes World No.8 Poland and 27th-ranked Senegal.
A victory against the African team in their next match on Sunday would guarantee Akira Nishino's side a place in the round of 16.
Meanwhile, 16th-ranked Colombia will need to defeat Poland, also on Sunday, just to keep alive their hopes of progressing to the knockout stage.
Colombia suffered a blow before Tuesday's match when Bayern Munich midfielder James Rodriguez was left out with a leg muscle problem.
As expected, the Cafeteros' all-time leading scorer Radamel Falcao was included in the starting eleven, having missed the 2014 World Cup with a knee injury.
Japan, who lost 4-1 to Colombia during the group stage in Brazil four years ago, included the experienced pair of Kagawa and Yuto Nagatomo in their starting lineup.
It took less than three minutes for the game to take a sharp turn in Japan's favor. Osako streamed forward on the counterattack and forced a reflex save from David Ospina, but the ball rebounded for Kagawa, whose shot was impeded by Carlos Sanchez's right arm.
Slovenian referee Damir Skomina pointed to the spot immediately and showed the Espanyol midfielder a straight red card.
Kagawa then sent Ospina the wrong way with his spot-kick to give the Blue Samurai a dream start.
Colombia manager Jose Pekerman introduced Boca Juniors holding midfielder Wilmar Barrios on the half-hour mark, sacrificing Juventus winger Juan Cuadrado, to give his team extra reinforcement as Japan threatened to double their lead.
But it was the South Americans who struck six minutes before the interval through River Plate playmaker Quintero. The 25-year-old cleverly fired under the Japan wall, beating a lunging Eiji Kawashima, who grasped the ball after it had already crossed the line.
Colombia defended resolutely in the second half as Japan sought to capitalize on their numerical advantage.
Rodriguez was brought on just before the hour mark, replacing Quintero, but the move did little to stop Japan's momentum.
Ospina saved shots from Osako and Takashi Inui before Maya Yoshida flayed an attempt wide as the Blue Samurai pressed relentlessly.
Hiroki Sakai then had a shot deflected behind after some slick build-up play. Substitute Keisuke Honda whipped in the resultant corner and this time Osako made it count, timing his leap to perfection to glance in off the far post.
Colombia continued to take the game to Japan and Rodriguez had a golden chance to equalize from the edge of the six-yard box but his shot was blocked by Osako.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
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