He studied pharmacy in Heidelberg and Berlin, and afterwards served as an assistant to Anton de Bary (1831-1888) at the University of Halle.
Brefeld was a prolific author of works in the field of mycology, being remembered for his writings on the heteroecious nature of fungal rusts and smuts.
[3] He is credited with providing nomenclature for a number of genera and species of fungi.
The names Conidiobolus, Heterobasidion, Oligoporus and Polysphondylium (genus of slime mold) are a few of genera that he described.
[4] Every two years, the Deutschen Gesellschaft für Mykologie offers the "Oscar-Brefeld-Preis" to young scientists for work in the field of mycology.