The "fair and balanced" Fox News host - who regularly slams the president on his no. 1 rated show - will write about Obama's "historical" administration in the book, set to be released in 2010.
"He's becoming a historical figure not because he's black, but because his liberal agenda is taking the country in a direction we've never seen before," O'Reilly said.
O'Reilly has most notably blasted Obama's economic recovery plan and bank bailout spending on his prime-time hit, "The O'Reilly Factor."
"I am skeptical of [Obama's] big government, nanny-state philosophy," O'Reilly wrote in a March 30 column on his Web site.
"The O'Reilly Factor" recently celebrated its 100th month as the no. 1 rated cable news show.
O'Reilly's latest book, "A Bold Fresh Piece of Humanity," has spent nearly six months on the New York Times best-seller list.