In 1973, O’Reilly enrolled at Harvard College to study classics and graduated cum laude with a Bachelor of Arts degree in 1975. During O’Reilly’s first year at Harvard, George Simon died in an accident.

After graduating, O’Reilly completed an edition of Simon’s Notebooks, 1965–1973. He also wrote a well-received book on the science fiction writer Frank Herbert and edited a collection of Herbert’s essays and interviews. O’Reilly got started as a technical writer in 1977. He started publishing computer manuals in 1983, setting up his business in a converted barn in Newton, Massachusetts, where about a dozen employees worked in a single open room. In 1989, O’Reilly moved his company to Sebastopol, California, and published the Whole Internet User’s Guide and Catalog, which was a best-seller in 1992. O’Reilly’s business, then known as O’Reilly & Associates, steadily grew through the 1990s, during which period it expanded from paper printed materials to web publishing. In 1993, the company’s catalogue became an early web portal, the Global Network Navigator, which in 1995 was sold to America Online.

In 1996, O’Reilly fought against a 10-Connection Limit on TCP/IP NT Workstations, writing a letter to the United States Department of Justice, Bill Gates, and CNN, concerned that the Internet is still in its infancy, and that limitations could cripple the technology before it ever has a chance to reach its full potential. In 2001, O’Reilly was involved in a dispute with Amazon.com, against Amazon’s one-click patent and, specifically, Amazon’s assertion of that patent against rival Barnes & Noble. The protest ended with O’Reilly and Amazon.com founder Jeff Bezos visiting Washington D.C. to lobby for patent reform.

In 1998, O’Reilly helped rebrand free software under the term open source. O’Reilly sees the role of open source as being inseparable from the development of the Internet, pointing to the widely used TCP/IP protocol, sendmail, Apache, Perl, Linux and other open source platforms. He is concerned about trends towards new forms of lock-in.

In 2001, O’Reilly coined the term inner source for the use of open source software development practices and the establishment of an open source-like culture within organisations whereby the organisation may still develop proprietary software but internally opens up its development.

In 2003, after the dot com bust, O’Reilly Media’s corporate goal was to reignite enthusiasm in the computer industry. Dale Dougherty, an executive at O’Reilly, invoked the phrase “Web 2.0” during a brainstorming session. Though O’Reilly is often credited with popularizing the phrase Web 2.0, it originated with Darcy DiNucci, who coined the term in 1999. O’Reilly went on to popularize the phrase as a handle for the resurgence of the web after the dotcom crash of 2000, and as a generic term for the “harnessing of collective intelligence” viewed as the hallmark of this resurgence. O’Reilly first called an “executive conference” in 2004, inviting five hundred technology and business leaders, followed by a public version of the event in 2005. Annual iterations of the event, known as the “Web 2.0 Summit” from 2006 onwards, continued until 2011.

The company suffered in the dotcom crash of 2000. As book sales decreased, O’Reilly had to lay off about seventy people, about a quarter of the staff, but thereafter rebuilt the company around ebook publishing and event production. In 2011 O’Reilly handed over the reins of O’Reilly Media to the company’s CFO, Laura Baldwin, but retained the title of CEO in recognition for the indispensable role he had in building the O’Reilly Media company and brand.

O’Reilly serves on the board of directors of three companies: Safari Books Online, Maker Media, and PeerJ. He served on the board of Macromedia until its 2005 merger with Adobe Systems, and on the board of MySQL AB until its sale to Sun Microsystems. He also serves on the board of directors for the advocacy group Code for America. In February 2012, he joined the UC Berkeley School of Information Advisory Board. As a venture capitalist, O’Reilly has invested in companies such as Blogger, Delicious, Foursquare, Bitly, and Chumby.

On 11 April 2015 O’Reilly married Jennifer Pahlka, a former colleague at O’Reilly Media, a former Deputy CTO of the US, and currently Founder and Executive Director of Code for America.

In 2017, O’Reilly’s book WTF? What’s the Future and Why It’s Up to Us was published, in which he discusses the consequences of technology and its potential to enhance the human experience.