The top seed needed three sets to overcome gutsy Russian Anna Blinkova 6-4 6-7 (5) 6-2.
Osaka was back on Arthur Ashe court for the first time since 'that' final, her victory 12 months ago which was completely overshadowed by Serena Williams' meltdown.
The 21-year-old is back at world number one despite poor showings at the last two grand slams, including a first-round exit at Wimbledon.
Osaka has been troubled by a knee injury - which was heavily strapped up - and even openly admitted she has not been enjoying playing tennis since her win in Australia in January.
She certainly did not enjoy the first three games, world number 84 Blinkova breaking her first service game to lead 3-0.
But Osaka won 11 points out of 12 on her way to clawing herself level before clinching the opening set.
Blinkova would not go away quietly, though and, having forged and squandered three set points at 6-5, she fashioned two more in the tie-break and converted the second.
It meant a deciding set Osaka could probably have done without given her fitness concerns, but an early break gave her some breathing space for the first time in the match.
A ferocious forehand winner which clipped the line wrapped up victory in a shade under two-and-a-half hours.
"It was really amazing for me, I have a lot of good memories here, I grew up here, and I wanted to add to that list of good memories," said Osaka.
"I don't think I've ever been this nervous in my life. She played amazingly and I never really found my rhythm.
"You want to do well after winning last year. I love playing here, that's why I played for nearly three hours! I definitely didn't want to lose in the first round.
"I learn from the tougher matches so it helps me be prepared."
Sloane Stephens, the champion just two years ago, crashed out to Russian youngster Anna Kalinskaya.
Kalinskaya, 20, won her first ever grand slam match 6-3 6-4 against the below-par American.
While former Wimbledon champion Garbine Muguruza was a first-round casualty as she went out in three sets to American Alison Riske 2-6 6-1 6-3.
Sixth seed Petra Kvitova, the two-time Wimbledon winner, eased through with a 6-2 6-4 victory over fellow Czech Denisa Allertova.
Kiki Bertens, the seventh seed, beat Paula Badosa of Spain 6-4 6-2.
Wimbledon champion Simona Halep was also taken to a deciding set.
The Romanian made it safely through, though, beating American Nicole Gibbs 6-3 3-6 6-2.
Halep said: "She started to play much better in the second set and I was dominated a bit but I refreshed my mind, came forward a bit more and that's how I got the win.
"Every grand slam is tough, every match is difficult. I'm happy to get through the first round because in the previous two years I lost in the first round."
Cori Gauff, the 15-year-old who lit up Wimbledon on her grand slam debut, has won her first match at the US Open.
Gauff dropped the first set to Russia's Anastasia Potapova but hit back to win 3-6 6-2 6-4.