WASHINGTON - U.S. President Donald Trump on Wednesday picked U.S. hostage negotiator Robert O'Brien as national security adviser, replacing hardliner John Bolton who was fired abruptly last week after clashing with the president on a host of issues.O'Brien, who will be the fourth person to hold the post in the Trump administration, has served as Trump's special envoy for hostage affairs at the U.S. Department of State since May 2018 and has a long history in Republican foreign policy circles. "I have worked long & hard with Robert. He will do a great job!" Trump wrote on Twitter.Trump fired Bolton after disagreements over handling foreign policy matters relating to North Korea, Iran, Afghanistan, Russia and Venezuela.O'Brien is an attorney from Los Angeles who has served as a foreign policy adviser to several Republican presidential campaigns, handled a number of high-profile legal cases and previously served in several State Department positions, including as an alternative representative to the U.N. General Assembly in 2005.Trump recently dispatched O'Brien to Sweden for the court hearing of U.S. rapper A$AP Rocky, who was later convicted for assault.Senate Republicans praised Trump's pick as a solid choice."He understands the world for the dangerous place it is. He's got great negotiating skills as our hostage negotiator," Senator Lindsey Graham told reporters. "He'll be a very sound policy adviser."John Barrasso, a member of the Senate Foreign Relations panel, told Fox News: "He will do a fine job."The president's previous national security advisers were H.R. McMaster, who was replaced by Bolton in March 2018, and Michael Flynn, who was fired shortly after taking the role and later pleaded guilty to lying to the FBI.