Mr. O'Brien, who has a five-book contract with Random House for a series of historical thrillers about the Lincoln assassination, oversaw the creation of Huffington., the site's iPad magazine that launched last year but didn't build up a subscription base and helped build up HuffPo's original reporting arm. Before going over  HuffPost in early 2011, Mr. O'Brien was the Sunday Business section editor at The New York Times. 

"After two years overseeing our national reporting staff in New York, Tim O'Brien is leaving to work on the second installment of his five-book publishing deal," Ms. Huffington wrote in the newsroom memo, obtained by Capital. "I'm truly grateful to him for helping us build such a robust and well-oiled original reporting infrastructure, including a crack team of news editors and copy editors, for helping launch the Huffington. magazine, and for editing David Wood’s series that won HuffPost a Pulitzer."

Mr. O'Brien's reporters will report to the editorial team of Howard Fineman, Peter Goodman, Ryan Grim, Kate Palmer, John Montorio, Sam Stein, and Amanda Terkel.