Sexual selection can act on all aspects of the phenotype and the opportunity for selection (I s) sets its maximal strength. Popular approaches to alter I s include the manipulation of the operational sex ratio (OSR) and/or density, with an increase in I s predicted with a male-biased OSR and at higher density. However, debate continues regarding the utility of I s to measure meaningful changes in the strength of selection, as changes in I s with OSR and density may only reflect stochastic processes. Here we tested whether the manipulation of OSR and density alters I s in the broad-horned flour beetle, Gnatocerus cornutus, a species where males are under intense sexual selection and the targets of selection are known. We also recorded the average number of fights and mating behaviour of individuals in our competitive arenas. We found significant main effects of OSR and density on I s, with the opportunity for selection being highest in male-biased high-density treatments. There were also significant effects of OSR and density on the average number of matings, whereas only density influenced the average number of fights. These results suggest that manipulation of OSR and density influence the opportunity for sexual selection in G. cornutus and our observations of fighting and mating behaviour provide a proximate mechanism for the change in I s.