Daniel Hosler O'Brien (born November 23, 1947, in Findlay, Ohio) is an American author, wildlife biologist, and rancher.

He attended Michigan Technological University to play football and graduated with a BS degree in Math and Business from Findlay College in 1970 where he was the chairman of the first campus Earth Day.

O'Brien continued to write and work as an endangered species biologist for the South Dakota Department of Game Fish and Parks and later the Peregrine Fund.

He now raises buffalo and lives on the Cheyenne River Ranch in western South Dakota with his wife Jill.

[citation needed] His books have been translated into seven foreign languages and his essays, reviews, and short stories have been published in many periodicals including, Redbook, New York Times Magazine, FYI.