U.S. President Donald Trump on Wednesday announced that his special presidential envoy for hostage affairs, Robert C. O’Brien will take on the role of national security advisor.

O’Brien will be Trump’s fourth national security advisor and replaces John Bolton who was ousted from the position on September 10.

“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 said on Twitter.

“I have worked long & hard with Robert. He will do a great job!”