* Left school as a teenager, became a billionaire
* Heads Norway's biggest private industrial group
* Says he suffers from "gigantomania"
OSLO, June 8 (Reuters) - He is the self-made man who changed the way business was done in Norway, and from this week, Kjell Inge Roekke, a flamboyant billionaire with an explosive temper and a taste for the supersized, can also call himself an oil baron.
His company Det norske, hitherto a rather modest oil explorer, paid $2.1 billion in cash to bag Marathon Oil's Norwegian business, turning it into the Nordic country's second-largest oil producer behind Statoil.
It is deals such as this that have turned Roekke, 55, into one of Norway's richest men, and turned his holding company, Aker ASA, into the country's largest private industrial group, with controlling stakes in Det norske, oil services firm Aker Solutions, heavy equipment maker Kvaerner, plus shippers and fisheries.
It has been a precipitous climb. Born in Molde, a small town on Norway's west coat, he left high school a dyslexic teenager with no qualifications and crossed the Atlantic to become a fisherman, catching pollock and crab from a base in Seattle.
He invested in a number of old boats, modified them into advanced factory trawlers, and so built his fortune. In the early 90s, he came back to Norway to shake up its sedate business world.
Roekke and a partner set their sights on one of Norway's venerable conglomerates, 173-year-old Aker, quickly bought up 40 percent of its shares in 1996, and merged it with their own Resources Group International, before snapping up Kvaerner four years later.
"He was the first one to bring American-style, aggressive capitalism to Norway, daring to use shareholder power to get what he wanted," said Steinar Dyrnes, a journalist at the Aftenposten daily who wrote a biography of Roekke.
"This was quite unheard of in Norway at the time," he said.
When Roekke seized Aker, many feared he would act like a corporate raider, gutting the company for cash. But that was not what drove him, then or now, his business associates say.
"He has shown quite the opposite," said Erik Haugane, a former chief executive at Det norske. "He has shown he thinks about the long-term when he makes investments and buys firms."
"He is a builder," said Kristin Krohn Devold, a former defence minister who sits on the Aker board with Roekke. "He gets a kick out of building things ... He loves building companies up, building industries, finding new products."
In a protestant, social-democratic country where egalitarianism trumps materialism, Roekke made waves with his glamorous girlfriends, private jet and flashy homes. There was even a pop song written about the heady mix of his business and party lifestyle.
His enduring passion is for boats of all shapes and sizes - but mostly large; he built the world's biggest fishing trawler and has made headlines with an ostentatious $90 million yacht, and a 217-foot sailboat. And for crashing in a speedboat race.
In 2007, he was convicted of bribing his way to a boat operating licence and served 23 days in prison. Even there the charismatic Roekke was something of a hit, and upon his release spent more than $3,000 on takeaway pizzas for his old cellmates.
"Roekke is an emotional person and he can express his emotions very strongly," said Dyrnes. "This means that he is often very charming and friendly. But there is another side. He can be very angry ... which is very un-Norwegian."
"The first time you experience his anger, it can be a bit of a shock," said Haugane, the former Det norske CEO. "You are thinking, 'What on earth is going on?' But then it passes. And the second and third time it happens, it is not as bad."
In 2009, the target of his anger was Industry Minister Sylvia Brustad, who claimed Roekke had bought and sold assets in a way that benefited himself but not other Aker Solutions shareholders, which include the government.
He hit back at a live news conference, claiming Brustad had got her facts wrong. The broadcast, which went on for hours, also turned personal, taking a swipe at her rural dialect.
And yet four years later she was hired by one of Roekke's companies to manage relations with the government.
"His strength is in coming up with lots of new, strategic ideas. But he needs someone to tell him which ones of his ideas are smart and which ones are not," said Dyrnes, the biographer.
At Aker, the man who helps him decide which ideas will work is chief executive Oeyvind Eriksen.
"It is true that we have complementary skills. We have a unique collaboration and relationship," said Eriksen.
Not all of Roekke's decisions look smart. In 1997 he bought into London's Wimbledon Football Club and later decided to relocate it to Milton Keynes, 55 miles (90km) north of its traditional home, an act of treachery to its fans. Record low crowds and poor results on the field sent the club into administration.
These days Roekke, who tends to decline media requests for interviews, communicates mostly through his letters to shareholders, published in Aker's annual reports.
In a letter published in the 2011 report, he admitted a penchant for the unusually and superfluously large.
"(My wife) Anne Grete is totally right when she says I suffer from an incurable disease: gigantomania. She has tried to cure me, but given up," he wrote.
"We had agreed to build a sailboat of 66. Anne Grete meant 66 feet; naturally, I was thinking in metres. It ended up at 66 metres."
He declined to say if he had giant plans for Det norske's new oil acquisition, but others in the company have said it will help finance development of its share of the $20 billion Johan Sverdrup oilfield in the North Sea - the biggest oil find off Norway in decades.
(Additional reporting by Terje Solsvik; Editing by Will Waterman)