Mrs. B. F. O’Connor was the international secretary-treasurer of the International Association of Machinists and served on the executive committee of the Flushing Branch of the Brooklyn Institute of Arts and Sciences. The Institute was later organized into the Brooklyn Museum.

Caroline O’Day (1869–1943) was the third woman, and first female Democrat, to be elected to Congress from New York. After studying in Paris, Munich, and Holland, O’Day served as the president of the school board in Rye, New York, and as a commissioner of the State Board of Social Welfare. She first became interested in politics during the suffrage movement and was an officer in the Westchester, New York, chapter of the League of Women Voters, where she first met Eleanor Roosevelt. From 1916 to 1920, O’Day served as the vice chairwoman of the New York State Democratic Committee and later as the associate chairwoman. She was elected to Congress in 1934, holding office from 1935 to 1943. O’Day’s platform stressed better wages and working conditions for laborers, strong support for federal intervention to relieve the effects of the Great Depression, and the need to involve women in local and national government. She was a staunch advocate for peace, working closely with the League for Peace and Freedom. O’Day cosponsored the Wagner-O’Day Act, passed in 1938, a U.S. federal law requiring that all federal agencies purchase specified supplies and services from nonprofit agencies employing persons who are blind or have other significant disabilities. She died the day after the end of her congressional service.

Kate Richards O’Hare (1877–1948) was a prominent anti-war activist involved in the socialist party during World War I. Born in Kansas, O’Hare moved to Kansas City where she became an apprentice to a machinist. She was so moved by a speech by labor activist Mary Harris Jones that she became an enthusiastic socialist. She joined the Socialist Labor Party in 1899 and later ran, unsuccessfully, for local office under the Socialist Party of America in 1910. During World War I, O’Hare traveled across the United States giving speeches as a member of the Socialist Party’s Committee on War and Materialism. In 1917 she was arrested under the Espionage Act and sentenced to five years in prison, though she was pardoned in 1920. She went on to organize a march where the children of socialists who were still imprisoned protested in Washington, D.C., creating public agitation to free their parents.

Claudia Ruth O’Keeffe (1899–1984) was the sister of American painter Georgia O’Keeffe.

Georgia O’Keeffe (1887–1986) was a contemporary painter whose depictions of the American landscape were in distinct contrast from the chaotic images of the art world’s avant-garde. Born in 1887, O’Keeffe studied at the Art Students League of New York as well the Art Institute of Chicago. She worked briefly as a commercial artist and then as a teacher in Texas. She picked up painting again after taking classes in teaching in 1915, exhibiting charcoal drawings at 291 gallery in New York by the following year. Two years later, O’Keeffe moved to New York City to focus on painting. Known for her large-scale paintings of flowers and cityscapes, O’Keeffe’s work depicted tension from New York to the American Southwest. She vacationed in Taos, New Mexico, in 1929, where Mabel Dodge provided her with a studio. In 1946, O’Keeffe moved permanently to New Mexico, the landscape of which appears extensively in her work. Despite feminist interpretations of her work as related to female genitalia, O’Keeffe such readings of her work and refused to participate in specifically feminist projects. The Whitney Museum held a retrospective for O’Keeffe in 1970, and her work is included in the collections of many major museums internationally.

Mary Kenney O’Sullivan (1864–1943) was a labor organizer and factory inspector. She became the first woman general organizer of the American Federation of Labor in 1892 and devoted her career to union organizing among women and to promoting protective legislation. She was also active on behalf of woman’s suffrage, child labor legislation, and prohibition. O’Sullivan was a member of Jane Adams’s settlement house movement and moved to Hull House in the 1880s where she continued to organize women’s work and clubs. She went on to found the Women’s Trade Union League in 1903 and assisted the International Workers of the World during the Lawrence Textile Strike. After the strike O’Sullivan helped pass legislation that would improve factory conditions in Massachusetts, eventually being hired by the Massachusetts Board of Labor and Industries in 1917 as an inspector, so she could help enforce the laws she helped to pass. She held that position until 1934, at which point she retired from labor organizing.

Violet Oakley’s (1874-1961) family were mostly artists. Consequently, she did not have to struggle to gain permission to study art and as a young girl in Bergen Heights, New Jersey. She was encouraged to attend the Art Students League, which she followed later with many trips to Europe. She described her infatuation with illustration as 'hereditary and chronic.' In 1902, Oakley was the first American woman to receive a public mural commission: "The Creation and Preservation of the Union," which consisted of a series of large murals for the walls of the Governor's Reception Room in the State Capitol Building in Harrisburg, Pennsylvania. During the first quarter of the twentieth century, she was renowned as a groundbreaker in mural decoration, a field that had been exclusively practiced by men. Oakley excelled at murals and stained glass designs that addressed themes from history and literature in Renaissance-revival styles. A committed artist of great integrity, she documented her works well, had numerous exhibitions, and worked up until the day she died in 1961.

Victoria Ocampo (1890–1979) was an Argentine writer and intellectual, known as the publisher of the legendary literary magazine Sur. One of the most prominent South American women of her time, she was a key figure of the intellectual scene in the 1920s and 1930s, publishing several books and the works of writers such as Gabriela Mistral, Jorge Luis Borges, and Julio Cortázar. During World War II she edited the anti-Nazi magazine Lettres Francaises in collaboration with her friend and translator Pelegrina Pastorino. Her open opposition to Peron’s regime in Argentina briefly landed her in jail. She became the first woman to ever be inducted into the Argentine Academy of Letters in 1976, soon after which the “cultural dialogue” initiated by UNESCO was held in her home, which she later donated to the organization.

Silvina Ocampo (1903–1993) was born to a wealthy family in Buenos Aires, the youngest of six sisters. After studying painting in Paris, she returned to Argentina where she devoted herself to writing and remained for the rest of her life. Her eldest sister, Victoria, was the founder of the seminal modernist journal and publishing house Sur, which championed the work of Jorge Luis Borges and Adolfo Bioy Casares. The first of Ocampo’s seven collections of stories, Viaje olvidado (Forgotten journey), appeared in 1937; the first of her seven volumes of poems, Enumeración de la patria (Enumeration of my country) in 1942. She was also a prolific translator—of Dickinson, Poe, Melville, and Swedenborg—and wrote plays and short stories for children. Ocampo won the second place prize in the National Poetry Competition in 1953 and first place in 1962.

Armen Ohanian (1887–1976) was an Armenian dancer, actress, writer, and translator. After a tumultuous childhood, including witnessing the anti-Armenian pogroms, she moved to Moscow in 1908. She performed her first dances at the Maly Theater. Ohanian traveled to Iran where she perfected her skills in Oriental dance and was one of the founders of the Persian National Theater in Tehran. After touring Egypt and the Ottoman Empire, she was hired to dance in London in 1911, and became a sought after performer as exotic dancers became a trend in Western circles. Using methods of “free dance” developed by American dancer Isadora Duncan, Ohanian created her own choreography based on Armenian and Iranian music. She performed extensively throughout Europe, relocating to Paris in 1912. After traveling to Mexico in the 1920s, she founded a dance school in Mexico City in 1936 and became an active member in the Mexican Communist Party.

Elizabeth Olds (1896–1991) was an American artist best known for her work in developing silkscreen as a fine art. She was primarily known as a printmaker, using silkscreen, woodcut, and lithography processes. In 1926, she became the first female honored with the Guggenheim Fellowship. Olds studied under George Luks and worked a Social Realist for the Public Works of Art Project in Omaha, Nebraska, and the Graphic Arts Division of the Federal Art Project in New York during the 1930s. Later in her career, Olds wrote and illustrated six children’s books, including Feather Mountain (1951), a runner-up for the Caldecott Medal. She was an artist-in-residence at the Yaddo and McDowell artist colonies in the 1950s and ’60s.

Olga Oppenheimer (1886–1941) was a German Expressionist artist who trained in Paris, Munich, and Dachau, while being encouraged by her family and provided with her own studio. She was a co-founder of Gereonsklub, a school and modern art venue in Cologne, which became a center of avant-garde art in Germany. Her work was exhibited in the International Sonderbund in Cologne in 1912 alongside the work of two other women artists, Marie Laurencin and Paula Modersohn-Becker. Oppenheimer was the only female German to be exhibited in the 1913 Armory Show. She entered a psychiatric institution for depression in 1918, where she spent the following twenty years. During World War II she was deported from Germany in 1941 to the Lublin concentration camp, where she died from complications with typhus.

Marjorie Organ (1886–1931) was one of the first female newspaper cartoonists in America, later moving into painting. Born in Ireland to a wallpaper designer, her family moved to America in 1899. Organ attended Saint Joseph’s School and the Normal College (later Hunter College) before studying at Dan McCarthy’s National School of Caricature. In 1902, at the age of sixteen, she joined the New York Evening Journal, for whom she created Reggie and the Heavenly Twins, possibly inspired by her friendship with Helen Marie Walsh. Organ’s other colorfully titled works included Girls Will Be Girls, The Man Haters’ Club and Lady Bountiful. Her commercial career ended in 1908, following a move to Connecticut, but the transition provided her the opportunity to develop an Impressionistic painting style that won her success. Organ exhibited work at the 1913 Armory Show in New York City, the Society of Independent Artists (1919–1924, 1926–1928), the McDowell Club (1917–1918), and the New York Society of Women Artists (1927, 1932).

Gerta Overbeck (1898- 1977) is known as a key figure in the German New Objectivity artistic movement that was active prior to the Second World War. The politics of the Weimar Republic, Communism, and the suffering German proletariat played an important role in Gerta’s life as an artist. In 1919, she went to study at the School for Artisans and Crafts in Hannover, Germany. She resolved her anti-establishment tendencies by joining the Community Party in the early 1920s. She worked in many fields, as a drawing teacher, as a factory worker, and in offices, like many of her peers, leaving painting to be done whenever free time was available. As a painter, Gerta was visually intrigued by construction sites and the traces of the industrial age. Not only was she interested in the daily struggles of the common man, she also wished her work to be accessible to them. Overbeck also contributed to a journal, Wachsbogen, with co-published writing that brought the young artists to the forefront. However in 1933, the Nazis came into power and paralyzed all artistic innovation. Greta Overbeck was rediscovered in the 1960s along with the rest of the New Objectivity movement. Her work has been included in most New Objectivity shows during the past sixty years.

Ruth Bryan Owen (1885-1954) served as a voluntary nurse in the Egypt-Palestine conflict during World War I. Upon her return she became a female pioneer in the film industry. She was a director, producer, and screenwriter for a feature film in 1922, called Once Upon a Time/Scheherazade. Owen had done extensive traveling after World War 1, and visited countries such as India, Burma, Sri Lanka, China and Japan. She became inspired by the places in her travels, and used that as the backdrop for her film. The film is thought to be lost today, and little would be known about it, had it not been for the correspondence between Owen and her dear friend, Carrie Dunlap. Owen demonstrated remarkable confidence and seemed to have little doubt in her ability to produce as well as to distribute an independent film from such an isolated position. In 1929 Owen became Florida’s (and the South's) first woman representative in the United States Congress in 1929, coming from Florida’s 4th district as a democrat. Representative Owen was also the first woman to earn a seat on the House Foreign Affairs Committee. In 1933, she became the first woman appointed as a U.S. ambassador to another country when President Roosevelt selected her to be Ambassador to Denmark and Iceland.