Clarke’s parents were a chest physician and a nursing sister. He was brought up near Canterbury with a brother and a sister. Clarke became head chorister at the Canterbury Cathedral Choristers School and subsequently won a choral scholarship to the independent school, The King's School, Canterbury. He later attended Pembroke College, Oxford where he studied theology and psychology. He also co-founded and captained the wine-tasting team. Clarke lives in West London.
Clarke started a career as a full time actor and singer working for the Royal Shakespeare Company, Royal National Theatre, The Old Vic and Chichester. In the West End, he played all the men in the Mitford Girls and Sweeney Todd.
He played a bank robber in the 1978 film Superman, a Special Branch man in Who Dares Wins, and Balthasar in Stuart Burge's 1984 film of Much Ado About Nothing as well as singing in the 1984 BBC adaptation of The Lord of The Rings.
In 1984 he became a full time writer. He joined the Sunday Express, where Lynn Barber initially was assigned as his ghostwriter. He then became wine correspondent of The Daily Telegraph.
In 1985, Clarke won the last World Wine Tasting Championship, making him reigning champion as the event has not yet been repeated.
Clarke has written several award-winning books, and is widely known for his partnership with Jilly Goolden as the wine experts on the BBC Two programme Food and Drink. More recently Clarke appeared on BBC Radio 4 in The Wine Programme with Andrew Jefford, in two series of Oz and James's Big Wine Adventure with James May, a sequel titled Oz and James Drink to Britain broadcast on BBC Two, and a subsequent christmas programme, Oz and Hugh Drink to Christmas.
In November 2008 a survey by the wine industry consultancy firm Wine Intelligence was made public, having polled the views of more than 1,500 regular UK wine drinkers. Results show that Clarke was the most recognised wine critic in the UK.
Oz was banned from the Champagne region of France in the 1990s for claiming that champagne was deteriorating in value whilst rising in price. This was revealed during Oz and James's Big Wine Adventure as he toured the Champagne region with James May.