Larry Ellison, a college dropout who built Oracle Corp. into one of America's largest and most prominent technology companies, is stepping down as chief executive in a momentous Silicon Valley handover.

Mr. Ellison, 70 years old, a brash personality who engaged in public fights with rivals and gobbled up potential competitors, has been the only CEO at Oracle. He founded the company in 1977 and through his canny pulse on technology and acquisitions turned it into a dominant seller of business software.

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