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 Julius Bierbaum (28 June 1865 – 1 February 1910) was a German writer.

Otto Liebmann was a German neo-Kantian philosopher.

Otto Ludwig was a German dramatist, novelist and critic born in Eisfeld in Thuringia. He was one of Germany's first modern realists and one of the most notable dramatists of the period.

Otto Müller (June 1, 1816 – August 6, 1894) was a German novelist.

Otto Karl Wilhelm Neurath was an Austrian-born philosopher of science, sociologist, and political economist. He was also the inventor of the ISOTYPE method of pictorial statistics and an innovator in museum practice. Before he fled his native country in 1934, Neurath was one of the leading figures of the Vienna Circle.

Otto of Sankt Blasien was a German Benedictine chronicler. He was born about the middle of the 12th century; died on 23 July 1223, at Sankt Blasien in the Black Forest, Baden. Nothing is known of the events of his life.

Otto Penzler is an American editor of mystery fiction, and proprietor of The Mysterious Bookshop in New York City.

Otto Pfleiderer was a German Protestant theologian. Through his writings and his lectures, he became known as one of the most influential representatives of liberal theology.

Otto Wilhelm Rahn was a German medievalist, Ariosophist, and SS officer who researched Holy Grail myths.