Oscar Fay Adams (1855–1919) was an American editor and author.

Oscar Devereaux Micheaux (; was an American author, film director and independent producer of more than 44 films. Although the short-lived Lincoln Motion Picture Company was the first movie company owned and controlled by black filmmakers, Micheaux is regarded as the first major African-American feature filmmaker, a prominent producer of race films, and has been described as "the most successful African-American filmmaker of the first half of the 20th century". He produced both silent films and sound films.

Oscar Fingal O'Fflahertie Wills Wilde was an Irish poet and playwright. After writing in different forms throughout the 1880s, he became one of the most popular playwrights in London in the early 1890s. He is best remembered for his epigrams and plays, his novel The Picture of Dorian Gray, and the circumstances of his criminal conviction for gross indecency for consensual homosexual acts in "one of the first celebrity trials", imprisonment, and early death from meningitis at the age of 46.

Oskar Bie was a German art historian and author of Jewish origin.

Oswald Arnold Gottfried Spengler was a German polymath, whose areas of interest included history, philosophy, mathematics, science, and art, as well as their relation to his organic theory of history. He is best known for his two-volume work, The Decline of the West, published in 1918 and 1922, covering human history. Spengler's model of history postulates that human cultures and civilizations are akin to biological entities, each with a limited, predictable, and deterministic lifespan.

Otis Adelbert Kline born in Chicago, Illinois, USA, was a songwriter, an adventure novelist and literary agent during the pulp era. Much of his work first appeared in the magazine Weird Tales. Kline was an amateur orientalist and a student of Arabic, like his friend and sometime collaborator, E. Hoffmann Price.

Ottilie Adelina Liljencrantz was an American writer of Norse-themed historical novels.

Jens Otto Harry Jespersen was a Danish linguist who specialized in the grammar of the English language. Steven Mithen described him as "one of the greatest language scholars of the nineteenth and twentieth centuries."

Otto von Kotzebue was a Russian naval officer in the Imperial Russian Navy. He commanded two naval expeditions into the Pacific for the purposes of exploration and scientific investigation. The first expedition explored Oceania, the western coast of North America and passed through the Bering Strait in search of a passage across the Arctic Ocean. His second voyage was intended as a military resupply mission to Kamchatka but again included significant explorations of the west coast of North America and Oceania.

Otto Weininger was an Austrian philosopher who lived in the Austro-Hungarian Empire. In 1903, he published the book Geschlecht und Charakter, which gained popularity after his suicide at the age of 23. Parts of his work were adapted for use by the Nazi regime. Weininger had a strong influence on Ludwig Wittgenstein, August Strindberg, Julius Evola, and, via his lesser-known work Über die letzten Dinge, on James Joyce.