On Oracle’s earnings call in mid-December, a Wall Street analyst asked whether the company was losing database customers to lower-cost alternatives. Oracle co-founder Larry Ellison insisted that its huge base of Fortune 500 clients, including two of its biggest customers, weren’t going anywhere. “Amazon continues to buy Oracle technology to run their business,” Mr. Ellison said, noting that Amazon spent $50 million on Oracle software during the quarter. Salesforce “runs entirely on Oracle,” he claimed.

But Mr. Ellison’s comments about Amazon and Salesforce don’t tell the whole story. In reality, both companies are actively working to replace Oracle software running on critical business systems with open-source database software alternatives, and have made significant progress toward getting off Oracle entirely, according to people familiar with the matter.