We encounter it, for example, when we t the cox model to survival data. The literature dealing with the relation between relative risk and odds ratio is quite exten. Sive some examples are davies et al. When two groups are under study or observation, you can use two measures to describe the comparative likelihood of an event happening.
Both are calculated from simple 2x2 tables. Some studies use relative risks rrs to describe results. Time remain silent as far as the occurrence of either exposure or event is considered. In fact, given only the minor allele frequencies in cases and controls and no counts, we cant actually compute the or or rr. The general rule though is that if the prevalence of the disease is risk and the odds ratio will be approximately the same. Or can be used to describe the results of case control as well as prospective cohort studies.
If risk of incidence is high in either or both exposed and unexposed, then risk ratio and odds ratio differ. The rarer the disease, the closer the approximation. An odds ratio is a ratio of ratios. The question of which statistic to use is subtle but very important.