Ortrud R. Oellermann is a South African mathematician specializing in graph theory. She is a professor of mathematics at the University of Winnipeg.

Oellermann was born in Vryheid.^[1] She earned a bachelor's degree, cum laude honours, and a master's degree at the University of Natal in 1981, 1982, and 1983 respectively,^[2] as a student of Henda Swart.^[3] She completed her Ph.D. in 1986 at Western Michigan University. Her dissertation was Generalized Connectivity in Graphs and was supervised by Gary Chartrand.^[2][4]

Oellermann taught at the University of Durban-Westville, Western Michigan University, University of Natal, and Brandon University, before moving to Winnipeg in 1996. At Winnipeg, she was co-chair of mathematics and statistics for 2011–2013.^[2]

With Gary Chartrand, Oellermann is the author of the book Applied and Algorithmic Graph Theory (McGraw Hill, 1993).^[AA]

She is also the author of well-cited research publications on metric dimension of graphs^[MD], on distance-based notions of convex hulls in graphs,^[CS] and on highly irregular graphs in which every vertex has a neighborhood in which all degrees are distinct.^[IG] The phrase "highly irregular" was a catchphrase of her co-author Yousef Alavi; because of this, Ronald Graham suggested that there should be a concept of highly irregular graphs, by analogy to the regular graphs, and Oellermann came up with the definition of these graphs.^[5]

In 1991, Oellermann was the winner of the annual Silver British Association Medal of the Southern Africa Association for the Advancement of Science.^[6] She won the Meiring Naude Medal of the Royal Society of South Africa in 1994.^[7] She was also one of three winners of the Hall Medal of the Institute of Combinatorics and its Applications in 1994, the first year the medal was awarded.^[8]