Over the past four decades, O'Brien has photographed subjects from presidents, celebrities, and financiers to small-town Texans, including ranchers, beauty queens, writers, and bar owners.

At college, he pursued a degree in Philosophy and became a photographer for the student paper, the Daily Beacon, earning four dollars for each published picture.

A turning point came when O'Brien met Jack Corn, a staff photographer for the Nashville Tennessean, and saw his documentary photography series on the coal mining community in Appalachia.

[8] In 1977, O'Brien won a second Robert F. Kennedy Journalism Award for "Culmer: The Tragic City", a photo essay of Miami's downtown ghetto.

In 1980, LIFE magazine published his ten-page black-and-white photographic essay capturing the heroic efforts of Nurse Charlotte Sheehan at the Burn Center in New York Hospital.

[11] O'Brien photographed subjects such as coal mining, Australian portraits, river oaks, and birding for feature stories in the National Geographic.

[16] For Apple's "What's on Your Powerbook Campaign", O'Brien photographed incongruous pairings of people holding laptops, including Todd Rundgren and Jesuit priest Don Doll.

The result earned a CLIO Award and was later named by Photo District News (PDN) one of the best ad campaigns of the last 25 years.

[16] Writing of O'Brien's aesthetic development through this period, Catherine Calhoun observed in Photo District News that O'Brien's "editorial work evolved in such a way that, by 1988, when he shot a pull-out, 28-image portfolio of Australians for National Geographic, his photographs had become thoughtfully composed pictures that combined passion with a hint of wit.

Notable subjects photographed by O'Brien include LeBron James, Steven Spielberg, Samuel L. Jackson, Bill Cosby, Al Sharpton, Philip Glass, Don DeLillo, Warren Buffett, Chris Evert, Troy Aikman, Larry McMurtry, Sissy Spacek, George W. Bush, and Donald Trump.

The book was a collaborative effort with former LIFE reporter Elizabeth Owen O'Brien (his spouse), who wrote brief, biographical stories to accompany each figure.

O'Brien's second book, Hard Ground, is a collection of black-and-white portraits of homeless individuals, accompanied by poetry from singer-songwriter and co-author Tom Waits.

"[6] To achieve the intimate effect he wanted, O'Brien used an old wooden-box view camera and Polaroid "Type 55" black-and-white film.

[6] "The abstract qualities of black and white," explains O'Brien, "convey emotion better and describe the subjects in a simple, elegant and consistent way.

[29] In 2010, O'Brien photographed the iconoclastic thinker, Charlie Munger, and over the next five years he traveled around the world taking portraits of prominent investors.

"[30] Michael O'Brien's photo prints are in the permanent collections of the Harry Ransom Humanities Research Center at the University of Texas at Austin, Birmingham Museum of Art, International Center of Photography in New York City, The Tennessee State Museum, and the Wittliff collections of Southwestern and Mexican Photography at Texas State University.