It is a measure of the strength of association between risk factor and outcome. The derivation of the Odds ratio is based on three assumptions:  a) The disease being investigated must be relatively rare. In fact, the majority of chronic diseases have a low incidence in the general population. b) The cases must be representative of those with the disease.  c) The controls must be representative of those without the disease. For example, the odds ratio is estimated at 8.1. This implies that the risk of oral cancer was 8.1 times greater in individuals who chewed tobacco than in those who did not chew tobacco.Soben Peter  Essentials of preventive and community dentistry  4th edition