Baboon researchers commonly use the timing of sexual swelling detumescence to infer the timing of ovulation. These estimates are then used for a variety of purposes, including the interpretation of male and female behaviour, assessment of likely paternity, and the calculation of gestation lengths. Although captive studies have measured the timing of ovulation with respect to detumescence, this has not been measured in wild baboons. Here, we use observational data on female sexual swellings, and hormonal data measured from faeces, to investigate the timing of ovulation with respect to detumescence in wild olive baboons. Our data show similar variability in the timing of ovulation with respect to detumescence as that produced by captive studies, although there are some notable differences, including the absence of any ovulations measured on or after the day of swelling detumescence. We discuss the importance of our results for baboon researchers using swellings to infer ovulation as part of their studies.