US President Donald Trump has named hostage negotiator Robert O"Brien to be his new national security adviser. Trump tweeted Wednesday that he has "worked long & hard" with O"Brien and that "he will do a great job!" Trump"s announcement about O"Brien comes a week after he ousted John Bolton from the national security adviser"s post, citing policy disagreements. Bolton was Trump"s third national security adviser. O"Brien has until now served as Trump"s envoy for situations involving US hostages abroad. 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 UN General Assembly in 2005. Trump had been weighing a number of potential replacements after Bolton left the post last week over his handling of North Korea and Venezuela. His 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.