Otto Schreier (1901–1929) was a German algebraist born in Vienna famous for his work in combinatorial group theory (Nielsen--Schreier method and HNN-extensions). His 1926 article on extensions of groups contains ideas of nonabelian cohomology and pseudofunctors; nowadays this is called Schreier’s theory of (nonabelian) extensions. The work has been completed by Eilenberg and Mac Lane in a series of three papers in the late 1940s. Higher analogues of Schreier’s theory were studied by L. Breen. Schreier’s theory can be also extended to some other algebraic structures, see e.g. nonabelian Lie algebra cohomology.
See biographical and some mathematical data at the MacTutor biography. For more on the mathematical side see