[1] Overgaard Nielsen was born at Snejbjerg in Herning, Denmark.
He investigated the role of the soil microfauna in the decomposition of plant litter and recycling of nutrients in ecosystems.
[2] He was a visiting scientist with Charles Elton at Oxford University from 1950-1953.
[4] He was employed by Molslaboratoriet at Aarhus University where he became department head in 1958 and associate professor in 1961.
[2][5] In 1964, he was appointed professor of zoology at the University of Copenhagen where he held the professorship until 1985.