The rugby player was brought in as a bombshell contestant - someone used to shake up the group already in the villa - shortly after the series began, when it emerged that he was an ex-boyfriend of Michael Owen's daughter Gemma who is also taking part in the show.
He then coupled up with Paige Thorne, but viewers were informed on Tuesday that he had quit Love Island on Sunday after a row about new contestant Adam Collard threatening to get between him and Thorne.
Speaking to The Sun about his time on the TV show, he said: “Doing Love Island was the worst decision of my life.”
O'Neill, 23, said he had become worried about his mental health and the potential for "things to go horribly wrong" if he stayed in the show, saying that the row with Collard, a contestant who first took part in the 2018 series and was brought back this year, had made him concerned that "I could get physical".
He added of his decision to quit: “I was ready to break down. I was feeling so mentally drained, I just wanted to go home and get myself right.
“It broke me and I was already broken. I was crying my eyes out. I couldn’t cope. I was literally feeling awful. It was the worst I’d ever felt in my life."
O'Neill also explained that producers had been aware of his ADHD diagnosis and that he felt it was starting to become difficult to cope with alongside the pressures of being on the programme.
He claimed that the show's producers had tried to convince him to go back into the villa, telling him "you’ll be fine", and said that he hadn't been given his phone back to call his mum for two days.
Viewers praised O'Neill for prioritising his mental health by choosing to go home.
Love Island has previously faced controversy as former contestants Sophie Gradon and Mike Thalassitis took their own lives after starring in the show and bosses have said that they have put in place rigorous safeguarding procedures in recent years.
Watch: Love Island's Adam and Paige couple up following Jacques' departure