robert charles o'brien (born june 18, 1966) is an american lawyer and politician. he has worked privately and with the united states government. he was the 28th united states national security advisor from september 18, 2019 to january 20, 2021. he also is an ambassador with the u.s. special presidential envoy for hostage affairs. he is a partner at larson o'brien llp, a los angeles-based boutique law firm.