O'Neil was born March 25, 1933 in Oklahoma City and raised in Texas. He studied business at Southern Methodist University, received a Bachelor's degree and served in the United States Air Force.
In 1960, he was accepted to Harvard Business School's first Program for Management Development (PMD).
In 1958 he started his career as a stockbroker at Hayden, Stone & Company, and developed an investment strategy which made early use of computers. He stated in a 2002 interview that one of the books which was an early influence on him was Gerald Loeb's The Battle for Investment Survival. According to O'Neil, this is the best book on the market. Other investors which he took great interest in were Bernard Baruch, Jesse Livermore, Jack Dreyfus, and Nicolas Darvas. He also greatly admired Thomas Edison.
From his research, O'Neil invented the CAN SLIM strategy and became the top-performing broker in his firm. He bought a seat on the NYSE at age 30 (the youngest at that time ever to do so), and in 1963 founded William O'Neil + Co. Inc., a company which developed the first computerized daily securities database in 1963/1964, and currently tracks over 200 data items for over 10,000 companies.
In 1973, he founded "O'Neil Data Systems, Inc.", to provide high-speed printing and database-publishing facilities.
In 1984, O'Neil made research from his database available in print form with the launch of Investor's Daily, a national business newspaper which has competed with The Wall Street Journal. In 1991, the publication's name was changed from Investor's Daily to Investor's Business Daily. Ten years after founding, it had a paid circulation of 149,557, with a claimed "total readership" of 850,000 (also phrased as "nearly 1,000,000 readers"), though in 2002, the Los Angeles Business Journal said that it has not been a moneymaker, and had had a decline in ad spending, though other O'Neil companies had done well.
O'Neil presently conducts investing seminars throughout the country.
