In the 1920s, in cooperation with his colleagues, Oort established the theory of the Milky Way's rotation, as well as deductions on its mass, and our Sun's distance from the center.

Oort is most widely known for his work in comet theory, however. In 1950, he proposed that a cloud of cometary material surrounds our solar system at an enormous distance, a concept that is now generally accepted. This cloud bears his name, The Oort Cloud.