Oort was born in Franeker in Friesland and studied in Groningen with Jacobus Cornelius Kapteyn. His Ph.D thesis was titled The stars of high velocity. In 1927 he proved that the Milky Way galaxy rotates, by analyzing the movements of stars. In 1935 he became professor in Leiden at the faculty where Ejnar Hertzsprung was the director.
Oort was fascinated by radio waves from the universe. After the Second World War he pioneered radio astronomy by using an old radar antenna from the Germans.
In the 1950s he raised funds for a new radiotelescope in Dwingeloo, in the east part of the Netherlands, to research the centre of the galaxy. In 1970 a bigger telescope was built in Westerbork, near the old old one. It consisted of twelve smaller telescopes working together.
His hypothesis that the comets have a common origin, postulated in 1950, was later proven to be correct. Another contribution of Oort was that he was able to demonstrate that the light from the Crab nebula was polarized.