President Donald Trump has named Robert O'Brien, the State Department's chief hostage negotiator, as his new National Security Adviser. The president made the announcement on Twitter on Wednesday. "I am pleased to announce that I will name Robert C. O'Brien, currently serving as the very successful Special Presidential Envoy for Hostage Affairs at the State Department, as our new National Security Advisor," Trump tweeted. Trump tweeted that he has worked "long & hard" with O'Brien, predicting he will do a "great job." O'Brien, a veteran on U.S. foreign policy matters, is Trump's fourth national security adviser during his two and a half-year-old presidency. O'Brien replaces John Bolton, whom Trump fired a week ago, saying that he committed some "big mistakes." According to the president, Bolton's stand on a number of foreign policy matters, including North Korean denuclearization, was not in line with his administration. Trump revealed in a post on Twitter last Tuesday that he dismissed Bolton. But the former U.N. Ambassador claimed that it was he who offered to resign ahead of the president's proposal. H.R. McMaster and Michael Flynn are the other National Security Advisers who lost their job under Trump.