Principal Officers and Chiefs of Mission, by Name: O

Note: Career Foreign Service appointees are asterisked.

• *Phyllis Elliott Oakley (1934–)
  • Assistant Secretary of State for Population, Refugees, and Migration Affairs (1994–1997)
  • Assistant Secretary of State for Intelligence and Research (1997–1999)
• *Robert Bigger Oakley (1931–2014)
  • Ambassador Extraordinary and Plenipotentiary, Congo (Kinshasa) (1979–1982)
  • Ambassador Extraordinary and Plenipotentiary, Somalia (1982–1984)
  • Director of the Bureau of Counterterrorism (1984–1986)
  • Ambassador Extraordinary and Plenipotentiary, Pakistan (1988–1991)
• James Curtis Oberwetter (1944–)
  • Ambassador Extraordinary and Plenipotentiary, Saudi Arabia (2003–2007)
• Edward Charles O’Brien (1860–1927)
  • Envoy Extraordinary and Minister Plenipotentiary, Uruguay (1905–1909)
  • Envoy Extraordinary and Minister Plenipotentiary, Paraguay (1905–1909)
• Thomas James O’Brien (1842–1933)
  • Envoy Extraordinary and Minister Plenipotentiary, Denmark (1905–1907)
  • Ambassador Extraordinary and Plenipotentiary, Japan (1907–1911)
  • Ambassador Extraordinary and Plenipotentiary, Italy (1911–1913)
• Vincent Obsitnik (1938–)
  • Ambassador Extraordinary and Plenipotentiary, Slovakia (2007–2009)
  • Ambassador Extraordinary and Plenipotentiary, Slovenia (Nomination of July 24, 2007, withdrawn on September 7, 2007.)
• Roderic L. O’Connor (1921–1982)
  • Assistant Secretary of State for Consular Affairs (1957–1958)
• *Daniel Anthony O’Donohue (1931–)
  • Ambassador Extraordinary and Plenipotentiary, Burma (1983–1986)
  • Ambassador Extraordinary and Plenipotentiary, Thailand (1988–1991)
• William O’Dwyer (1890–1964)
  • Ambassador Extraordinary and Plenipotentiary, Mexico (1950–1952)
• Benjamin Hilborn Oehlert (1909–1985)
  • Ambassador Extraordinary and Plenipotentiary, Pakistan (1967–1969)
• *Paul Christopher O’Friel (1959–)
  • Chargé d’Affaires ad interim, Iceland (2015)
• Andrew Jackson Ogle (1822–1852)
  • Chargé d’Affaires, Denmark (1852)
• *John Martin O’Keefe (1946–)
  • Ambassador Extraordinary and Plenipotentiary, Kyrgyzstan (2000–2003)
• *Herbert Stuart Okun (1930–2011)
  • Ambassador Extraordinary and Plenipotentiary, German Democratic Republic (1980–1983)
• John Callan O’Laughlin (1873–1949)
  • Assistant Secretary of State (1909)
• *Herbert Vincent Olds (1913–1999)
  • Chargé d’Affaires ad interim, Burundi ()
• Robert Edwin Olds (1875–1932)
  • Assistant Secretary of State (1925–1927)
  • Under Secretary of State (1927–1928)
• John O’Leary (1947–)
  • Ambassador Extraordinary and Plenipotentiary, Chile (1998–2001)
• Covey Thomas Oliver (1913–2007)
  • Ambassador Extraordinary and Plenipotentiary, Colombia (1964–1966)
  • Assistant Secretary of State for Western Hemisphere Affairs (1967–1968)
• Louise Oliver (1944–)
  • U.S. Permanent Representative to the United Nations Educational, Scientific and Cultural Organization (2004–2009)
• *Mary Seymour Olmsted (1919–2018)
  • Ambassador Extraordinary and Plenipotentiary, Papua New Guinea (1975–1979)
  • Ambassador Extraordinary and Plenipotentiary, Solomon Islands (1978–1979)
• Richard Olney (1835–1917)
  • Secretary of State (1895–1897)
• *Arthur Joseph Olsen (1920–2003)
  • Chargé d’Affaires ad interim, Sweden (1974)
• *Clinton Louis Olson (1916–)
  • Ambassador Extraordinary and Plenipotentiary, Sierra Leone (1972–1974)
• Jack B. Olson (1920–2003)
  • Ambassador Extraordinary and Plenipotentiary, The Bahamas (1976–1977)
• Lyndon Lowell Olson Jr. (1947–)
  • Ambassador Extraordinary and Plenipotentiary, Sweden (1997–2001)
• *Richard Gustave Olson Jr. (1959–)
  • Ambassador Extraordinary and Plenipotentiary, United Arab Emirates (2008–2011)
  • Ambassador Extraordinary and Plenipotentiary, Pakistan (2012–2015)
• Kevin Francis O’Malley (1947–)
  • Ambassador Extraordinary and Plenipotentiary, Ireland (2014–2017)
• *Adrienne S. O’Neal (1954–)
  • Ambassador Extraordinary and Plenipotentiary, Cape Verde (2011–2015)
• Emmet O’Neal (1887–1967)
  • Ambassador Extraordinary and Plenipotentiary, Philippines (1947–1948)
• *Joseph P. O’Neill (1935–2014)
  • Chargé d’Affaires ad interim, Eritrea ()
• John D. Ong (1933–)
  • Ambassador Extraordinary and Plenipotentiary, Norway (2002–2005)
• *John Malcolm Ordway (1950–)
  • Ambassador Extraordinary and Plenipotentiary, Armenia (2001–2004)
  • Ambassador Extraordinary and Plenipotentiary, Kazakhstan (2004–2008)
  • Chargé d’Affaires ad interim, Kazakhstan (2014)
• John Davis O’Rear (1868–1918)
  • Envoy Extraordinary and Minister Plenipotentiary, Bolivia (1913–1918)
• Bruce J. Oreck (1953–)
  • Ambassador Extraordinary and Plenipotentiary, Finland (2009–2015)
• *Cristobal Roberto Orozco (1944–)
  • Chargé d’Affaires ad interim, Brazil (2002)
• James Lawrence Orr (1822–1873)
  • Envoy Extraordinary and Minister Plenipotentiary, Russia (1872–1873)
• Robert D. Orr (1917–2004)
  • Ambassador Extraordinary and Plenipotentiary, Singapore (1989–1992)
• William Horsley Orrick Jr. (1915–2003)
  • Deputy Under Secretary of State for Management (1962–1963)
• Godlove Stein Orth (1817–1882)
  • Envoy Extraordinary and Minister Plenipotentiary, Austria (1875–1876)
• *Francis Vincent Ortiz Jr. (1926–2005)
  • Ambassador Extraordinary and Plenipotentiary, Barbados (1977–1979)
  • Ambassador Extraordinary and Plenipotentiary, Grenada (1977–1979)
  • Ambassador Extraordinary and Plenipotentiary, Guatemala (1979–1980)
  • Ambassador Extraordinary and Plenipotentiary, Peru (1981–1983)
  • Ambassador Extraordinary and Plenipotentiary, Argentina (1983–1986)
• *David Lawrence Osborn (1921–1994)
  • Ambassador Extraordinary and Plenipotentiary, Burma (1974–1977)
• Thomas Andrew Osborn (1836–1898)
  • Envoy Extraordinary and Minister Plenipotentiary, Chile (1877–1881)
  • Envoy Extraordinary and Minister Plenipotentiary, Brazil (1881–1885)
• Thomas Ogden Osborn (1832–1904)
  • Minister Resident, Argentina (1874–1884)
  • Minister Resident/Consul General, Argentina (1884–1885)
• John Eugene Osborne (1858–1943)
  • Assistant Secretary of State (1913–1916)
• *Lithgow Osborne (1892–1980)
  • Ambassador Extraordinary and Plenipotentiary, Norway (1944–1946)
• Nelson O’Shaughnessy (1876–1932)
  • Chargé d’Affaires ad interim, Mexico (1914)
• *Elim O’Shaughnessy (1907–1966)
  • Chargé d’Affaires ad interim, Hungary (1966)
• *Theodore G. Osius III (1961–)
  • Ambassador Extraordinary and Plenipotentiary, Vietnam (2014–2017)
• *Nancy Ostrander (1925–)
  • Ambassador Extraordinary and Plenipotentiary, Suriname (1978–1980)
• John Louis O’Sullivan (1813–1895)
  • Chargé d’Affaires, Portugal (1854)
  • Minister Resident, Portugal (1854–1858)
  • Envoy Extraordinary and Minister Plenipotentiary, Portugal (Nomination of February 25, 1856, withdrawn before the Senate acted upon it.)
• Maria Otero (1950–)
  • Under Secretary of State for Civilian Security, Democracy, and Human Rights (2009–2013)
• William Joseph O’Toole (1894–1928)
  • Envoy Extraordinary and Minister Plenipotentiary, Paraguay (1922–1924)
• Marcus Otterbourg (1827–1893)
  • Chargé d’Affaires ad interim, Mexico (1867)
  • Envoy Extraordinary and Minister Plenipotentiary, Mexico (1867)
• Mary Martin Ourisman (1946–)
  • Ambassador Extraordinary and Plenipotentiary, Saint Vincent and the Grenadines (2006–2009)
  • Ambassador Extraordinary and Plenipotentiary, Saint Lucia (2006–2009)
  • Ambassador Extraordinary and Plenipotentiary, Saint Kitts and Nevis (2006–2009)
  • Ambassador Extraordinary and Plenipotentiary, Barbados (2006–2009)
  • Ambassador Extraordinary and Plenipotentiary, Grenada (2006–2009)
  • Ambassador Extraordinary and Plenipotentiary, Antigua and Barbuda (2006–2009)
  • Ambassador Extraordinary and Plenipotentiary, Dominica (2006–2009)
• *Henry David Owen (1920–2011)
  • Director of Policy Planning (1966–1969)
  • Ambassador at Large (1978–1981)
• *Michael S. Owen (1951–)
  • Chargé d’Affaires ad interim, Tanzania (2005)
  • Ambassador Extraordinary and Plenipotentiary, Sierra Leone (2010–2013)
• Robert Dale Owen (1801–1877)
  • Chargé d’Affaires, Kingdom of the Two Sicilies (1853–1854)
  • Minister Resident, Kingdom of the Two Sicilies (1854–1858)
  • Envoy Extraordinary and Minister Plenipotentiary, Kingdom of the Two Sicilies (Nomination of February 25, 1856, withdrawn before the Senate acted upon it.)
• Roberts Bishop Owen (1926–2016)
  • Legal Adviser (1979–1981)
• Ruth Bryan Owen (1885–1954)
  • Envoy Extraordinary and Minister Plenipotentiary, Iceland (1933)
  • Envoy Extraordinary and Minister Plenipotentiary, Denmark (1933–1936)
  • Envoy Extraordinary and Minister Plenipotentiary, Denmark (1933)
• *Barbro Appelquist Owens-Kirkpatrick (1946–)
  • Ambassador Extraordinary and Plenipotentiary, Niger (1999–2002)
• Alvin Mansfield Owsley (1888–1967)
  • Envoy Extraordinary and Minister Plenipotentiary, Romania (1933–1935)
  • Envoy Extraordinary and Minister Plenipotentiary, Ireland (1935–1937)
  • Envoy Extraordinary and Minister Plenipotentiary, Denmark (1937–1939)
• Stephen A. Oxman (1945–)
  • Assistant Secretary of State for European and Eurasian Affairs (1993–1994)