We examined how manipulating operational sex ratio (OSR: the ratio of reproductively active males to fertilizable females), could affect the intensity of competition in reindeer (Rangifer tarandus) using an experimental approach to test mating systems theory on the very first field study on large mammals of its kind. Study was conducted at the Kutuharju Reindeer Research Station where we had access to a herd of semi-domesticated reindeer. We manipulated OSR on two levels in two enclosures, a female biased treatment (3♂:6♀ = OSR 0.5) and a sex balanced treatment (3♂:3♀ = OSR 1). Female reindeer were injected with 1 ml of estrumate prior to a trial to promote early and synchronized oestrous to assure competition among our 1.5-year-old males. We predicted that with increasing OSR, male-male aggression would increase and courtship behaviours would decrease, unless males adopted alternative mating tactics. Conversely, female-female aggression was predicted to decrease with increasing OSR. The propensity for males to act agonistically was predicted to increase with increasing OSR. Male-male rate of aggression did not differ between treatments. Female-female rate of aggression was found to decrease significantly with increasing OSR. Propensity to act agonistically was not significantly different between treatments. 1.5-year-old males may be too young and inexperienced to behave predictably. We conclude that the intensity of competition cannot be predicted at the group level based on OSR alone, though some trends emerged.