When Thomas Kurian, a high-profile former Oracle executive, left the software company last September, his peers at the database software company expected Oracle to replace him.

But Larry Ellison, Oracle’s co-founder and chief technology officer, had other plans. Mr. Kurian had been leading Oracle’s response to a growing threat from Amazon, which offers online alternatives to Oracle’s database products and other software. In a meeting shortly after the departure of Mr. Kurian, Mr. Ellison surprised his inner circle by saying he was excited to personally take over the duties of the former product development leader, according to a former Oracle employee with direct knowledge of his comments.

Almost 42 years after he helped create Oracle, Mr. Ellison, 74, has entered a late-career act unlike that of any of his contemporaries. While many of the tech industry’s founding fathers have moved on to full-time philanthropy (Microsoft’s Bill Gates) or died (Apple’s Steve Jobs, Intel’s Andy Grove), Mr. Ellison has rolled up his sleeves and hopped into the trenches of product development. This account is based on interviews with more than two dozen former Oracle employees and people close to the company.