Galactic motion
Oort used nearby stellar motions to study the Milky Way’s rotation and refine the Sun’s position within it, connecting local observations to the structure of the larger stellar system.
References: [1]
A proposed comet reservoir
Many long-period comet orbits pointed to origins far beyond Pluto. Oort inferred a surrounding comet reservoir from these orbital patterns. The cloud is an indirect inference rather than a directly photographed structure, and he was not the first to suggest a distant comet swarm.
References: [1]
Leiden and radio astronomy
Director of Leiden Observatory from 1945 to 1970, he promoted Dutch radio observatories at Dwingeloo and Westerbork. He also served as general secretary and later president of the International Astronomical Union.
References: [1]