Osius, a career diplomat, disagreed with Trump administration policy. While in office, he opposed deporting Vietnamese people who arrived in the United States before July 12, 1995 — the date the two former foes re-established diplomatic relations — and were protected from deportation by an agreement both countries signed in 2008. The Trump administration reinterpreted the agreement in the spring of 2017 so that Vietnamese people convicted of crimes were exempt from its protections; the administration now says the agreement fails to prohibit the deportation of all noncitizen Vietnamese who arrived in the U.S. before 1995.

The former ambassador expressed his dissent in letters and diplomatic cables to officials, including Secretary of Defense Jim Mattis, then-Secretary of State Rex Tillerson and then-national security adviser H.R. McMaster.