Trump announced the appointment of O'Brien, who was serving as special presidential envoy for hostage affairs at the State Department under Secretary Mike Pompeo, on Twitter. The position does not require Senate confirmation.
"I have worked long and hard with Robert," Trump said. "He will do a great job."
O'Brien, Trump's fourth national security adviser since taking office, replaces the hawkish Bolton, a harsh Iran critic who had a rocky relationship with other Cabinet members before leaving the job last week.
O'Brien served as the U.S. alternate representative to the United Nations General Assembly under President George W. Bush in 2005-06. He was the founding co-chair of the Department of State Public-Private Partnership for Justice Reform in Afghanistan from 2007 to 2011 under Secretaries of State Condoleezza Rice and Hillary Clinton.
He worked as a senior legal officer for the U.N. Security Council in Geneva from 1996 to 1998, where he addressed claims against Iraq that came out of the first Gulf War. O'Brien served as a major in the U.S. Army Reserve JAG Corps.