The 'Young' singer and the Skins actor are believed to have parted ways over the weekend after three months together.
Adressing the break-up on Twitter, Contostavlos wrote: "4 the record I didn't dump any1, it was a mutual decision with no hard feelings n that's all there is 2 it."
The pair were rumoured to have been dating since mid-March, and had been spotted together on several occasions.
While Contostavlos had confirmed she was in a relationship, the pair had never publicly announced that they were an item.
They allegedly broke up after she flew over to Bulgaria where O'Connell was filming his latest movie.
An insider told The Sun: "With them being so far away from each other, it just wasn't working."
X Factor judge Contostavlos had previously split from former N-Dubz bandmate Fazer back in January.
Watch Tulisa Contostavlos's video for 'Young' below: