O’Brien has previously compared the 2015 Iran nuclear deal to the 1938 Munich Agreement that allowed Adolf Hitler to annex the Sudetenland.
In his book While America Slept, which has been described as a “wake-up call for the American people”, O’Brien lambasted Barack Obama’s “lead-from-behind” foreign policy, which he said had made the world more dangerous.
He said the country needed a “return to Reagan’s ‘leader of the free world’ foreign policy and a ‘peace through strength’ national security approach”.
The 2016 book featured an endorsement on the back cover from Bolton, who was fired by Trump last month following a series of disagreements with the president over North Korea, Iran, Afghanistan, Russia and Venezuela.
As special envoy for hostage affairs, O’Brien has worked on releasing US prisoners in Iran, and helped to secure the return of Danny Burch, an American hostage in Yemen.
Trump wrote on Twitter: “I have worked long & hard with Robert. He will do a great job!”
The former lawyer from Los Angeles was described by one US official as the “nicest guy on the planet”, and someone who would not cause disruption.