Trump forced Bolton, a leading foreign policy hawk, out last week, saying he "strongly" disagreed with his national security adviser on multiple issues.
"I have worked long and hard with Robert", Trump tweeted in his announcement of the replacement. "He will do a great job!" posted the President on social media.
O'Brien most recently was in the limelight during the trial of A$AP Rocky in Sweden, where he was sent by Trump as an observer.
When someone like Bolton comes in with a policy agenda to implement behind the scenes, it can do a president a disservice, said Loren DeJonge Schulman, who served at the National Security Council and Pentagon. His appointment was announced by President Trump via Twitter, whilst in the city for a campaign fundraiser.
Daniel Runde, senior vice president of the Center for Strategic and International Studies, reviewed O'Brien's book for foreign policy and featured him in his 2017 podcast "Building the Future".
Senate Republicans praised Mr O'Brien as a solid choice.
He served as Co-Chairman of the United States Department of State's Public-Private Partnership for Justice Reform in Afghanistan, launched in December 2007 by Secretary of State Condoleezza Rice.
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'He ripped Trump, without using his name, several times, ' one attendee said.
He was given the rank of ambassador one year later.
But O'Brien also was involved in other cases including that of Danny Burch, who as working as an engineer at an oil company when he was taken hostage by a criminal gang in Yemen.
He said he would advise Mr Trump privately on the situation in Saudi Arabia.
In his 2016 book While America Slept, O'Brien criticised what he called then outgoing president Barack Obama's attempt to present a more collaborative, dovish United States.
Peace talks with the Taliban and a surprise planned meeting between the insurgents and Trump himself, something apparently opposed by Bolton, were scrapped earlier this month.
The policy differences between Trump and Bolton were no secret, and Trump said the final straw happened when Bolton wanted to apply to "Libya model" to the USA handling of North Korea.
"Upon taking the job, O'Brien will become the highest-ranking Mormon in the USA government, an important development for a religious community that has shown some skepticism of Trump and will be a closely watched voting demographic in states such as Arizona", the article says. The new national security adviser will also take a leading role on addressing crisis in Venezuela, as well as the president's efforts to get North Korea to denuclearize. "The stakes in this unsafe time could not be any higher".