President Trump named Robert C. O’Brien on Wednesday as his new national security adviser, tapping a UC Berkeley Law School alum who grew up in the Bay Area for the influential position.

O’Brien will replace John Bolton, who left the White House last week. Trump said he asked for Bolton’s resignation, while Bolton said he offered it up.

O’Brien graduated in 1991 from UC Berkeley Law School after earning his undergraduate degree from UCLA. His LinkedIn page indicates he’s an alumnus of Cardinal Newman High School in Santa Rosa. He graduated in 1984, according to KGO-TV.

O’Brien, an attorney and Army Reserve veteran, has been serving as the special presidential envoy for hostage affairs at the State Department — the nation’s top hostage negotiator. Trump has taken a special interest in retrieving U.S. citizens held as hostages abroad, and said in a tweet announcing O’Brien’s appointment that the two had worked closely together.

“I am pleased to announce that I will name Robert C. O’Brien, currently serving as the very successful Special Presidential Envoy for Hostage Affairs at the State Department, as our new National Security Advisor. I have worked long & hard with Robert. He will do a great job!” Trump tweeted.

I am pleased to announce that I will name Robert C. O’Brien, currently serving as the very successful Special Presidential Envoy for Hostage Affairs at the State Department, as our new National Security Advisor. I have worked long & hard with Robert. He will do a great job! — Donald J. Trump (@realDonaldTrump) September 18, 2019

Less than an hour before, Trump had tweeted he had ordered “substantially” higher sanctions on Iran.

The national security adviser is not a Senate-confirmed position, serving at the discretion of the president. O’Brien will take the position amid heightened tensions with Iran, concerns about a nuclear North Korea, continued instability in Venezuela and uncertain election results in Israel.

Tal Kopan is The San Francisco Chronicle’s Washington correspondent. Email: tal.kopan@sfchronicle.com Twitter: @talkopan