"I was really pissed off," O'Neill sighs. "After all, I've drunk enough of their product over the years. It should be payback time. Guinness saw it was a show about alcoholism and thought 'negative'. But in Edinburgh, six people came up to me after the show and said, 'I have a drink problem and that's the first time I've been able to laugh at it. You've given me the courage to do something about it'. That was brilliant."
A man with a family history of drink problems, O'Neill admits that thanks to his alcoholism, "I was quite mad for a number of years. I hit rock bottom when I realised I was going to lose my family. My three-year-old son was beginning to get frightened of me. He wouldn't come in to see me because of the stink of drink. I thought, 'I'm putting this kid through what I've gone through'.
"I also got sick of waking up in the morning and not knowing where I'd been or who I'd insulted. I once had a blackout for three days. I began drinking with pounds 300 on a Friday night in Fulham and woke up on a beach in Brighton on Monday morning with no shoes, socks or money, and bruised from head to foot. I've never found out how I got there. I could easily have woken up in a police station and been told 'you've killed someone'. 'Really? I can't remember'. That was scarey stuff and the moment I realised it was time to knock the drinking on the head."
These experiences have been turned into a brutally honest show which often teeters on the tightrope between comedy and tragedy. Indeed, O'Neill recalls that "people said, 'you can't lay this on people, it's not fair'. At first I thought maybe they were right, before realising that it made me laugh, so to hell with them."
If you can believe it, the show could have been even heavier. "I only scratched the surface. I held back. There was much much worse in real life, but if I put it in the show it would have been too dark for the audience."
There was a time when "Off My Face" would have been too dark for O'Neill, too. "It would have been painful if I'd written it in the first year after giving up drinking. But now I haven't drunk for seven years, and I feel completely detached from it. I'm writing about someone else, I was a very different person then. When I wrote it, I wanted to make sure I was definitely off the drink. I didn't want to go on tour with this show and start drinking again."
Own O'Neill's 'Off My Face' is at the Bloomsbury Theatre, London WC1 (0171 388 8822) from Wed to Sat
It takes a bold - or perhaps foolhardy - performer to riff on just one subject for a whole hour. Phill Jupitus is one of the few stand- ups who could pull it off. "Ready Jedi Go" is his inspired ramble on the theme of Star Wars. The Essex man makes a pilgrimage home to the Basildon Towngate Theatre (01268 531343) on Thursday