It was the final track Ariana performed on Monday.
The song is being streamed and downloaded after a fan campaign to get the track to the top of the official charts.
The track has climbed from outside the top 200 overnight to number 35 and it's currently outselling Justin Bieber and Liam Payne on iTunes.
However iTunes sales no longer contribute greatly to chart positions, with the top 40 more influenced by listeners' streaming habits.
"A number one on the iTunes Chart is unlikely to reflect the UK market overall across all retailers and streaming services," a spokesman for the Official Charts company told Newsbeat.
The video for One Last Time has been watched more than 286 million times on YouTube since its release in 2015.
One Last Time was the fifth single from Ariana's 2014 album My Everything and originally reached number 24 in the UK charts when it was released as a single.
The singer has cancelled the remaining dates on her Dangerous Woman tour following the terror attack.
Her management say her live shows have been "suspended until we can further assess the situation and pay our proper respects to those lost".