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In the evolutionary biology of sexual reproduction, operational sex ratio is the ratio of sexually competing males that are ready to mate to sexually competing females that are ready to mate, or alternatively the local ratio of fertilizable females to sexually active males at any given time. This differs from physical sex ratio which simply includes all individuals, including those that are sexually inactive or do not compete for mates.
The theory of OSR hypothesizes that the operational sex ratio affects the mating competition of males and females in a population. This concept is especially useful in the study of sexual selection since it is a measure of how intense sexual competition is in a species, and also in the study of the relationship of sexual selection to sexual dimorphism. The OSR is closely linked to the "potential rate of reproduction" of the two sexes; that is, how fast they each could reproduce in ideal circumstances. Usually variation in potential reproductive rates creates bias in the OSR and this in turn will affect the strength of selection. The OSR is said to be biased toward a particular sex when sexually ready members of that sex are more abundant. For example, a male-biased OSR means that there are more sexually competing males than sexually competing females.