In Los Angeles, fans placed flowers on Newton-John's Hollywood star as those who had worked with her throughout the years shared memories and condolences on social media.
John Travolta, who played Newton-John's love interest in "Grease," shared an emotional tribute on Instagram. "My dearest Olivia, you made all of our lives so much better," he wrote. "Your impact was incredible. I love you so much."
Another of her "Grease" co-stars, Barry Pearl -- who played T-Bird Doody -- broke down as he remembered his friend during an interview with the Today Show on CNN affiliate 9.
"Everything sounds so trite when you discuss who she is and what she was, but it is so because it's so profoundly true ... a heart that was so giving and loving," Pearl said.
"Grease" casting director Joel Thurm became emotional when asked how he would remember the star, saying she was "kind, absolutely wonderful, and courteous to everybody."
Newton-John's death is being felt especially deeply in Australia, where she grew up. Born in England, she moved with her family to Melbourne, Australia at age 5 and as a teen, she appeared on weekly pop music shows in the country.
In a tweet, Australian Prime Minister Anthony Albanese called Newton-John "a big part of the Australian soundtrack."
Australia House in London said Newton-John, who was made a dame by the Queen in 2019 for her services to charity and cancer research, brought Britons and Australians together. "Dame Olivia inspired us in song and brought us together as one," the official account tweeted.
The Olivia Newton-John Cancer Wellness and Research Centre in Melbourne, where flags were set at half-staff Tuesday, said in a statement, "Olivia touched the lives of many people across Australia and the world, but none more so than our cancer services staff and patients at the Olivia Newton-John Centre, who she encouraged, inspired and supported every day."
Kylie Minogue, one of Australia's most successful musicians whose early career was often compared to Newton-John's, shared a photo of the two of them from the Royal Bicentennial Concert in 1988. "Since I was ten years old, I have loved and looked up to Olivia Newton John. And, I always will ... She was, and always will be, an inspiration to me in so many, many ways," Minogue tweeted.
Australian singer and actress Delta Goodrem, who played Newton-John in a 2018 TV biopic, posted a heartfelt tribute on Twitter.
"The whole world will feel this heartbreak today because the entire world felt Olivia's unmatched light. A force for good. A force of nature. Strong and kind. My mentor, my friend, my inspiration, someone who always guided me... always there. Family to me.... I love you forever," Goodrem shared with a photo of her and Newton-John together.
Oscar-winning actress Marlee Matlin took to Twitter to honor Newton-John, her co-star in the 1996 movie "It's My Party." "I am SO saddened at the news of the passing of Olivia Newton John. I remember being so star struck when I met her at my first Hollywood gathering for Paramount. She was the sweetest and brightest light and I loved getting to know her on 'It's My Party.' RIP dear, sweet Olivia," Matlin wrote.
Dionne Warwick, who recorded a duet with Newton-John in 2006, called her a "dear friend" and "one of the nicest people" she had the pleasure of recording and performing with. "Another angelic voice has been added to the Heavenly Choir," she added.
Star Wars actor Mark Hamill tweeted, "Everything she did was infused with a joyousness that was absolutely contagious. We loved her & she loved us. #RIP_OliviaNewtonJohn."
Julianne Hough, who played Sandy in the 2016 TV remake "Grease Live!," shared a few photos of herself and Newton-John on Instagram, writing, "Like so many, Olivia has always been my hero. Discovering her magnetic energy as an artist and actress came first but it was her heart that truly captured mine. Forever an icon and legend!"
On Twitter, Oprah Winfrey recalled "a spontaneous dinner" she hosted in 2019 after an interview about Newton-John's cancer diagnosis. In the post, Oprah noted "her positivity was just infectious."
Jane Lynch, who sang Newton-John's hit song "Physical" with her on "Glee," tweeted simply, "ONJ. Angel."
RuPaul's Drag Race shared a picture from Newton-John's time as a guest judge for an episode of the show in 2015, tweeting, "Rest in peace, Olivia Newton-John. Thank you for sharing your talent and inspiring us all."