Monnig was born in Fort Worth, Texas, the United States. In 1925 he received a law degree from the University of Texas. He worked for the family dry goods business and was its president from 1974 to 1981, when it was sold.
In the 1920s Monnig started to be interested in astronomy. He founded the Texas Observers astronomy club and between 1931 and 1947 he published a monthly newsletter – the Texas Observers Bulletin – writing especially about issues of amateur astronomers' interest, such as variable stars, meteors, comets, and the planets.
In the late-1920s, Monnig developed an interest in meteorites and their contribution to astronomers' studies on the origins of the Solar System. He was a founding member of the Society for Research on Meteorites (later renamed Meteoritical Society).
In the early-1930s he started his own meteoritical collection. After he failed with his requests to be allowed to study meteorites at the Smithsonian Institution, the Field Museum, and the American Museum of Natural History, he increased his collecting effort.
He questioned witnesses of meteoroid detonations or bolides and organized and financed searching expeditions. He paid 1 dollar per pound, a price that museums could not match at the time of the Great Depression.
Gradually, his collection grew to be one of the largest private collections in the world: it contained about 3,000 specimens from 400 different meteorites. Probably the most valuable were two carbonaceous chondrites, found at Crescent, Oklahoma, in 1936 and at Bells, Texas, in 1961.
Monnig later decided to donate the collection to Texas Christian University: he made a series of transfers, between 1976 and 1986. Nowadays, the collection contains over 1,000 different meteorites. In 2003, four years after he died, the Oscar E. Monnig Meteorite Gallery was opened, exhibiting about 10 per cent of the meteorites to the public.
In 1984, Monnig became the first person to win the Texas Star Party's annual 'Lone Stargazer Award' for his accomplishments as an observer in amateur astronomy  In 1990, he received the Amateur Achievement Award of the Astronomical Society of the Pacific for his contributions to meteoritics. The main belt asteroid 2780 Monnig was named in his honour.
- Williams, Thomas R. (2000). "Obituary: Oscar E. Monnig, 1902-1999". Bulletin of the American Astronomical Society 32 (4): 1682–1683. Bibcode:2000BAAS...32.1682W.
- Ehlmann, Arthur J.; Timothy J. McCoy. "A biography of Oscar Monnig". Texas Christian University. Retrieved 2007-08-12.
- "Welcome to the Oscar E. Monnig Meteorite Gallery". Texas Christian University. Retrieved 2007-08-12.
- "Past Amateur Achievement Winners". Astronomical Society of the Pacific. Retrieved 2007-08-12.
|Amateur Achievement Award of Astronomical Society of the Pacific