Odd Olai Aalen (born 1947) is a Norwegian statistician and is a professor at the Department of Basic Medical Sciences at the University of Oslo. His research work is geared towards applications in biosciences. Aalen's early work on counting processes and martingales, starting with his 1976 Ph.D. thesis at the University of California, Berkeley, has had profound influence in biostatistics. Inferences for fundamental quantities associated with cumulative hazard rates, in survival analysis and models for analysis of event histories, are typically based on the Nelson–Aalen estimator or appropriate related statistics. The Nelson–Aalen estimator is related to the Kaplan-Meier estimator and generalisations thereof.
Aalen is currently leader of the Centre for Biostatistical Modelling in the Medical Sciences at the Faculty of Medicine at the University of Oslo. He is an elected member of the Norwegian Academy of Science and Letters.