Otto is remembered for one book, and it distorts him. He was a working comparativist who travelled a great deal at a time when most German theologians did not, through North Africa, Egypt, Palestine, India, China and Japan, and much of his output is an attempt to test whether the category he had described in Das Heilige turns up outside Christianity.
His conclusion was that it does. The study he published in 1926 setting the Advaita Vedanta of Shankara beside the German mysticism of Meister Eckhart argues that two thinkers separated by language, century and religion arrive at strikingly parallel structures, and that the parallels are too specific to be accidental. He also produced work on the Bhagavad Gita and on the devotional theism of Ramanuja and Indian bhakti, insisting against the fashion of his day that Indian religion was not exhausted by monism.
He proposed, without much success, an interreligious body he called the Religious League of Mankind, on the model of the League of Nations, which would speak jointly on moral questions without doctrinal agreement. He assembled a collection of religious objects from his travels which became the museum of religion at Marburg, one of the first of its kind. He died in 1937 after a fall from a tower, in circumstances that have never been fully clarified.