The pair were close friends.
The 38-year-old dedicated part of his performance at the O2 Arena in London to his friend, who passed away in 2020.
In honour of Flack, Olly sang a rendition on Neil Diamond’s ‘Sweet Caroline’ – but was seen holding back tears on stage.
Speaking to the crowd, the star revealed that he has “regrets” about not being there for Caroline, who tragically took her own life at the age of 40.
He said: “Like I said earlier, it’s been an incredible four or five years and I know it’s been a really tough, crazy four or five years for every single one of you in this room.
“It’s going to be hard for me to do this but, in 2020, I lost a very good friend of mine, someone that I love very, very much.
“Her name is Caroline Flack and it is one of the toughest things that has happened in my life and it is really sad to see someone so amazing leave us but she has never left and she is always in my heart”.
Olly went on to say that he has “so many questions that I wanted answered and I am never ever going to get them answers now”.
At the start of his tour last month, Olly vowed to dedicate the song to his “dear friend” Caroline at every show he ever performs.
In an interview after coming off stage, he said: “It’s hard to sing that in front of 12,000 people. I love Caz and it was my way of remembering her legacy, who she was in my life. I always said I don’t want anyone to forget what an amazing person she was”.