The video also included a link to a fund in Olivia’s name to raise money for brain tumour research.
The teenager was diagnosed with brain cancer two years ago and recorded the song on Sept. 6. Unable to stand, Olivia can be seen sitting at a microphone as she sings a softer version of the song.
The video became so popular that Katy herself caught wind of it and felt compelled to share a heartfelt message with the teen.
"I just wanted to reach back out to you and tell you that I saw your video, and I was very moved. You sounded great," Katy says in a video response.
"I love you, a lot of people love you and that's why your video got to me and it moved everybody that saw it. I just wanted to send you some love and some light and tell you that I'm thinking about you.
Olivia reportedly saw Katy’s message and was very emotional.
Watch Katy’s message below.