Mr O'Brien has given all the credit for hostage releases he worked on to Mr Trump. "He will do a great job", he said.
The announcement of O'Brien's selection comes a week after the Republican president ousted John Bolton from the national security adviser's post, citing policy disagreements.
O'Brien had previously worked with Bolton during George W. Bush's administration.
Trump personally dispatched O'Brien to help free United States rapper A$AP Rocky from Swedish prison earlier this year. He works closely with the families of American hostages and advises the senior leadership of the U.S. Government on hostage issues. Another short-listed candidate, North Korea envoy, Stephen Biegun, was taken out of the mix when Pompeo suggested he might be better placed as Deputy Secretary of State to replace John Sullivan, who is widely expected to be nominated to be the next US ambassador to Russian Federation, officials said.
One of the most prominent hostage cases O'Brien has worked on is that of Austin Tice, a freelance journalist believed by the US government to be alive who was abducted in Syria in August 2012 while reporting on the uprising against President Bashar al-Assad.
That said, O'Brien did recently spend some time on screens. Trump tweeted from California, where he is holding fundraising events.
"Robert O'Brien said Trump is the greatest hostage negotiator in history".
O'Brien, though, has far less national security or foreign policy experience than past national security advisers - under Trump or otherwise.
O'Brien is an attorney from Los Angeles who served as a foreign policy adviser to 2012 Republican presidential nominee Mitt Romney and 2016 candidate Scott Walker.
He has also been the only USA official consistently working on releasing Americans imprisoned in Iran.
Interested in Trump Administration? O' Brien has a law degree from the University of California, Berkeley, and co-founded a law firm in Los Angeles focused on litigation and global arbitration issues.