President Trump has tapped Robert O'Brien to be his new national security adviser, replacing ousted former national security adviser John Bolton. O'Brien is currently the State Department's special presidential envoy for hostage affairs.

O'Brien had been among the top candidates for the position. Mr. Trump announced his pick in a tweet Wednesday morning.

Speaking to reporters in California, Mr. Trump said he got to know O'Brien very well through his work for hostages, calling him a "very talented man."

Get Breaking News Delivered to Your Inbox

"Mr. O'Brien is highly respected," the president told reporters. "He was highly respected by so many people that I didn't even know really knew him. He did a tremendous job on hostage negotiation. Really tremendous. Like, unparalleled. We've had tremendous success in that regard, brought home many people. And through hostage negotiation I got to know him very well myself but also a lot of people that I respect rated him as their absolute number one choice. So you know I think we have a very good chemistry together and I think we're going to have a great relationship. He is a very talented man."

O'Brien's arrival comes moments after the president announced new, ambiguous sanctions on Iran, after drone attacks on a Saudi oil facility resulted in an upheaval of the world's oil supply chain.

Senior State Department sources tell CBS News O'Brien was Pompeo's top pick for the position. The two are longtime friends, and Pompeo advocated heavily for the selection of O'Brien. Pompeo made no secret of the fact that he clashed at times with O'Brien's predecessor, John Bolton.

The position of national security adviser doesn't require Senate confirmation.

O'Brien was responsible for helping with the A$AP Rocky case in Sweden, after the rapper was found guilty of assault but did not face prison time for it.

As national security adviser, O'Brien will be responsible for consolidating sensitive information for the president and presenting him with options. Mr. Trump openly said he had serious disagreements with Bolton, and that's why he was pushed out of the position last week.

Immediately, O'Brien will be tackling any U.S. response to the Saudi oil strikes. Secretary of State Mike Pompeo is scheduled to be in Saudi Arabia the next couple days to assess the situation. O'Brien will also be in the position to give Mr. Trump guidance on what to do in Afghanistan, which was a major point of contention between Mr. Trump and Bolton.