Trump had said Bolton had been a "disaster" on North Korea policy, "out of line" on Venezuela, and did not get along with important administration officials.
O'Brien, who now serves as the special presidential envoy for hostage affairs at the State Department, has a long history in Republican foreign policy circles. "He will do a great job!" the president tweeted from his fundraising trip in California.
"Trump, after enduring the Bolton experience, clearly has no use for a strong personality or ideologue in the National Security Adviser position".
O'Brien previously co-chaired the State Department's justice reform initiative in Afghanistan, as well as advised the government on the trafficking of antiquities, according to his biography.
Earlier this year, Mr Trump sent him to Sweden to monitor the assault court case of American rapper A$AP Rocky, in which the president took a personal interest.
"O'Brien joined the administration in May 2018 and serves as the Special Presidential Envoy for Hostage Affairs at the State Department".
O'Brien has helped free US hostages including Danny Burch, an engineer at an oil company who was taken hostage by a criminal gang in Yemen.
"The president has had unparalleled success in bringing Americans home without paying concessions, without prisoner exchanges, but through force of will and the goodwill that he's generated around the world", O'Brien said at the time. In 2005, Bush nominated O'Brien to be U.S. Representative to the United Nations General Assembly, where he worked with Bolton.
The position of national security adviser doesn't require Senate confirmation.
In his 2016 book, While America Slept, O'Brien detailed what he saw as some of the failures of US foreign policy in recent years, particularly under the Obama administration.
He also was an adviser on the Republican presidential campaigns of former Governor Scott Walker of Wisconsin, former Governor Mitt Romney of MA and Senator Ted Cruz of Texas. He was a major in the U.S. Army Reserve.
Twenty-five pilotless aircraft and cruise missiles were used to attack the two sites, Maliki told reporters gathered in Riyadh. Saying the Houthis were responsible for the drone strikes, he said: "They attacked an industrial center to warn you".
Those financing and trade-in options may be enough to fight both the current phone market saturation and the iPhone's steep costs. The iPhone 11 Pro comes with a 5.8-inch OLED display in a body that's no bigger than the iPhone 7 which packs a 4.7-inch display.
UAW's leadership had previously won overwhelming approval from its rank-and-file for a strike if it became necessary. Thousands of UAW members went on strike at the General Motors manufacturing complex in Flint at midnight on Monday.
Trump affirmed he was ready to help its ally Saudi Arabia after the attacks that triggered a record leap in world oil prices. Mr Khamenei told state television: "Iranian officials, at any level, will never talk to American officials".
The event invitation itself is sparse on details other than confirming a 90Hz display which seems to becoming a new trend. It may include the same camera setup as of OnePlus 7T expect it's telephoto lens with improved 3x optical zoom support.