THE Burren Energy founder collected £67m when a £1.74bn cash bid by Italian energy giant Eni succeeded in 2007.
Burren had floated on the London exchange five years earlier, valued at about £175m. O'Sullivan founded the oil exploration and shipping company in 1994.
A qualified geophysicist, after graduating from UCG in 1977 O'Sullivan worked for firms including Chevron, United Geophysical, Geographical Systems and Olympic Oil.
He credits his success to a willingness to work in challenging regions, from freezing Canada to pirate-infested waters near Nigeria.
He based himself in Britain in 1991, to be closer to the new oil opportunities in the former Soviet Union.
He realised there was a shortage of reliable shipping from the Caspian region so he started a business specialising in exporting crude. He was also exploring oil and gas projects in Turkmenistan and later the Republic of Congo.
These dual interests morphed into Burren Energy, a company he initially ran from his garage in Hampshire.
Burren grew its projects in regions such as Turkmenistan, Russia and West Africa and floated in 2003. Forbes noted that its revenue more than doubled in 2005 to £260m.
When it was bought out in 2007 it was producing 37,000 barrels a day and its share price had soared to over £12.
The 55-year-old lives in Hampshire with his wife Diana. The couple have three daughters.