Osaka’s 73rd minute header was the difference between the sides as the Japanese made the most of their one-man advantage over the South Americans, who saw Carlos Sanchez sent off with just three minutes on the clock.
Shinji Kagawa gave Japan the lead from the penalty spot only for Juan Quintero to level the scores six minutes before the break.
But Nishino’s side took the initiative in the second half and, after creating a string of chances, Osako pounced 17 minutes from time to give Japan their first World Cup win since a 3-1 triumph over Denmark in 2010.
"I'm thrilled because I realised a lifelong dream of scoring at the World Cup,” said the 28-year-old striker. “We knew they - not just us - were going to be nervous at the start of the game and we managed to capitalise on that.
"We've been working tirelessly on our set-pieces and executed just as we did in practice. Everyone deserves credit for this win. But we still have two more games to play."
Kagawa was nerveless from the spot after a three-minute wait following Sanchez’s expulsion and he calmly slotted the penalty home to put Japan in front.
In the process, he buried memories of his failure from 12 yards in the quarter-finals of the AFC Asian Cup quarter-finals in 2015, when his miss against the United Arab Emirates saw Japan eliminated.
And after today's results, Group H looks like this...
1) #JPN
2) #SEN
3) #POL
4) #COL #WorldCup pic.twitter.com/GRHNBY8ga4— FIFA World Cup (@FIFAWorldCup) June 19, 2018
"I'm incredibly relieved,” said the Borussia Dortmund midfielder. “The penalty was huge. It's a good thing I was able to take it calm and cool. I was just trying to throw the 'keeper and it worked out well, just as I've been doing in training. I'm really happy about that.
"Everyone understood the importance of the first game and we ran our hearts out for this victory. I'm so glad we won. We really believed in ourselves, the entire squad, to produce the result we have. But we have to sharpen up and refocus for the next two games.
"We haven't accomplished anything yet. Starting from recovery tomorrow, we need to start preparing for our next match."
The Japanese take on Senegal in their next Group H match with the teams sharing top spot after the African side defeated Poland 2-1 later on Tuesday. As a result, a win over Aliou Cisse’s side could be enough to take Japan into the knockout phase for the third time in the country’s history.
Keisuke Honda was instrumental in Japan’s run to the Round of 16 in South Africa in 2010 and he played a role in this win, coming off the bench to deliver the cross from which Osako scored the winner.
"I did what I was asked to do, which was to help create the deciding goal so I'm really pleased about that,” he said. “We managed to win three points so we have to be happy about that.
"But I can't say the same about our performance. As everyone saw, they were down to 10 men for 80 minutes or so and I don't think you can say we made the most of our advantage.
"No one will rate you without results so how we conduct ourselves from today will matter even more than ever. We're still in a position where we have to win at least one of our next two.
"But I think we did well to put ourselves in such a position. If we win once more, we're in a good spot and we earned that - and I think we can pat ourselves on the back for that."
Source: the-afc.com