Few people in the high-tech industry have feuded as openly as Oracle's flamboyant CEO Lawrence Ellison and Thomas Siebel, the co-founder and chairman of rival enterprise software maker Siebel Systems.

When Siebel, a longtime Oracle exec, launched his self-named software company in 1993, he set out to create an anti-Oracle of sorts. He had angrily quit Oracle several years earlier, as Siebel told the story, because Ellison couldn't see the potential in selling software for automating customer call centres. Siebel took that idea and turned it into a billion-dollar company, beating Oracle to what turned out to be huge market for software that manages a corporation's customer relationships.

But in the end, Oracle's sheer bulk, the slowdown in enterprise software spending, and the move toward buying giant suites of software like the package Oracle sell -- rather than so-called best-of-breed software like Siebel's -- meant apparent victory for Oracle. On Monday, Oracle announced that it will buy Siebel in a deal worth US$5.8 billion.

Not long ago, an Oracle-Siebel deal was hard to imagine, and if history is a guide it's still difficult to believe Tom Siebel will stick around to work for his old boss once the deal is completed. If Siebel did move back to Oracle headquarters, just five miles down the road from Siebel central on Silicon Valley's Highway 101, it would be a remarkable detente for two very different men.

Where Oracle had a reputation for a technology-first pitch to customers, Siebel believed his company should go to customers first, ask what they want and build it for them. The discount brokerage Charles Schwab was among Siebel's first customers, and Charles Schwab himself was for years a member of Siebel's board of directors.

Many consider Oracle, whose first customer was the Central Intelligence Agency, Silicon Valley typified, with fancy sports cars in the parking lots and employees eschewing old-fashioned corporate dress codes. On the other hand, Siebel wanted his company to be conservative -- even a little stodgy -- to better suit its big, often-stodgy customers.

But bringing Siebel Systems into the growing Oracle empire started seeming a lot more plausible in the summer of 2004, when court documents introduced in Oracle's antitrust fight with the Justice Department revealed that Siebel and Ellison had already talked about combining their companies.

It's not clear what role, if any, Tom Siebel will have at Oracle once the acquisition is completed. Siebel has not run day-to-day operations at his company since May 2004, but he was considered an active chairman, still involved in strategic planning. Analysts believe he played a key role in the April ousting of Mike Lawrie, a former IBM executive who was briefly Siebel's chief exec.

Ellison said on a conference call with investors on Monday that Siebel was welcome to stay with the company, but didn't say in what role. Siebel didn't address the issue on the call, and many analysts doubt he'll stick around. "I don't think you're going to see much of Tom Siebel inside Oracle," said Gartner analyst Michael Maoz.

The 61-year-old Ellison's bio reads more than a little like "The Great Gatsby." He is a well-respected yachtsman who has competed in the America's Cup, won the deadly Sydney-to-Hobart race off the coast of Australia and has lavished a reported US$80 million of his own money on his yacht-racing team.

Ellison has embraced -- critics would say craved -- the limelight, even allowing "60 Minutes II" to see his nearly completed, 23-acre compound in Woodside, California, which was built to look like a Japanese palace. Ellison brought in Japanese stonemasons to make the giant rocks in a manmade waterfall look as though they "were placed there by the hand of God over the last million years," as Ellison put it on "60 Minutes."

He's also wildly wealthy. Ellison is worth US$18.4 billion, making him No. 9 on Forbes magazine's annual list of the wealthiest people in the world. He counts among his closest friends Apple Computer CEO Steve Jobs, and he was an active supporter of Bill Clinton. Perhaps the only difference between Ellison and Gatsby is a noticeable lack of a ruinous Daisy Buchanan in his life. The playboy adventurer is now on his fourth wife.

Meanwhile, back at the ranch
Siebel, on the other hand, has eschewed the limelight. Like Ellison, the 51-year-old is a licensed amateur pilot and is famously competitive. He's also rich. Siebel made it into the Forbes list at his company's peak, and stands to become even wealthier thanks to the Oracle deal. But he's a major donor to the Republican Party who was regularly hobnobbed with Republican powerbrokers like Marc Racicot, a Siebel board member who three years ago was appointed head of the Republican National Committee.

Siebel also as studiously avoided Ellison's flamboyance. While Ellison tends to stay inside an executive suite, replete with a jewel-encrusted statue of Buddha that's hundreds of years old, Siebel preferred to rub elbows with his employees, often eating lunch in a corporate cafeteria side-by-side with engineers. The only real indication of wealth in his office was a wall-covering William Alexander Coulter painting of the evacuation of San Francisco after the city's 1906 earthquake and fire.

Siebel routinely told reporters in interviews that they could ask him anything, but even the most innocent of questions about his family were off-limits. Still, he was embarrassed by press reports three years ago that he threw, at his own expense, a lavish birthday party for his wife in Puerto Vallarta, Mexico, just as his company was running into trouble.

While Ellison revels in being a Silicon Valley powerbroker, by many accounts Siebel prefers to keep to himself. He's an avid golfer, and these days spends much of his time on a sprawling ranch in Montana. Siebel worked as a ranch hand for a brief time after college, and often said that relaxation to him was doing the hard ranch work you don't find very often in corporate offices.

The childhoods of the two executives were different, too. Though they both grew up in Illinois, Ellison was raised in the blue-collar south side of Chicago, and is famously a college dropout. Siebel was raised in a comfortable Chicago suburb, attended military school and actually graduated from college. He even has an MBA.

Still, despite Siebel's aspirations, his company and Oracle weren't all that different. Both had extremely aggressive sales forces headed by executives whose personalities were inseparable from the companies they created. Culturally, analysts say, it's not as bad a fit as some would think.

But it's hard to imagine that Oracle, as massive as it is thanks to its recent acquisition spree, is big enough for both the personalities of Larry Ellison and Tom Siebel.