William O'Neil is an American entrepreneur, stockbroker, and author, best known for founding the investment research firm Investor's Business Daily. He developed the CAN SLIM strategy, which combines fundamental and technical analysis to help investors identify growth stocks. O'Neil gained prominence in the 1980s with his book, How to Make Money in Stocks, where he shares his investment philosophy and techniques. His approach emphasizes the importance of earnings growth, market trends, and proper timing in stock trading, making it popular among individual investors and traders.