After dropping the first four games of Tuesday's encounter on Rod Laver Arena, the ninth-seeded American found her groove, going on to win 7-6 (8-6) 6-1.
Twelve months ago, Keys was celebrating a career-defining and long-awaited first major title after upsetting Aryna Sabalenka on the very same court.
"No matter what you think it's going to be like, how you try to prepare yourself, when you do all of that, you at least know that you have done your part to be ready for the occasion," said the 30-year-old Keys.
"But the moment they say, 'ready, play', it kind of hits you in a way that I don't think you can ever really explain.
"But as nerve-wracking and as stressful as that can be, I'm still reminding myself of just how few people get to be in that moment, and being able to walk out today and have the crowd be as welcoming as they were.
"I'll take that stress any day."
On her grand slam debut, the 25-year-old Oliynykova jumped out of the blocks and raced to a 4-0 lead before Keys clicked into gear after coming to terms with the Ukrainian's moonball-style groundstrokes.
"They're so high and they're so deep, so it was really hard," said Keys.
"I felt like I couldn't really take a swing volley or take it off the bounce just because she hits such a good ball off of that.
"So I felt like I just had to wait for the right opportunities."
Sporting a wide array of permanent tattoos - plus some temporary ones of flowers on her face - Oliynykova's appearance was as eye-catching as her playing style.
The world No.95 proved a big hit with the crowd on Rod Laver Arena.
"For me, it's the best experience of my career," she said.
"I've never even played a WTA250 main draw before and I've never played on such a big arena.
"This experience is something I'll remember until the end of my life."
Keys will play fellow American Ashlyn Krueger in the second round.
In other early women's action on Tuesday, Janice Tjen upset former US Open finalist and �Canadian 22nd seed Leylah Fernandez 6-2 7-6 (7-1) to become the first Indonesian to win a match at the Australian Open in 28 years.
Fifth-seeded 2023 AO runner-up and 2022 Wimbledon champion Elena Rybakina from Kazakhstan beat Slovenia's Kaja Juvan 6-4 6-3, while Swiss 10th seed Belinda Bencic continued her unbeaten start to the year downing Brit Katie Boulter 6-0 7-5.
In a clash between two highly-credentialled unseeded players, former world No.1 Karolina Pliskova from Czechia ousted 2017 US Open champion Sloane Stephens 7-6 (9-7) 6-2.
Former French Open champion Jelena Ostapenko also advancing beating Slovakia's Rebecca Sramkova 6-4 6-4.
