Oum Sang-il (Korean: 엄상일; born 1976) is a Korean mathematician working in graph theory and discrete mathematics. He is a tenured professor in the Department of Mathematical Sciences at KAIST and the chief investigator of the Discrete Mathematics Group in the Pioneer Research Center for Mathematical and Computational Sciences at the Institute for Basic Science. He is known for his work on structural graph theory and in particular for structures and algorithms relating to rank-width, clique-width, and branch-width. He published more than 45 journal papers.