Odds ratio: It is the ratio of the products of those events that were expected divided by those events that were unexpected.
I am first going to cite an example for this. See a group of people exposed to a risk factor like spoilt burgers in a party. Those who ate burgers and got gastroenteritis and those who did not eat and did not get gastroenteritis are Expected outcomes. On the other hand are the unexpected - those who ate and did not get gastroenteritis and those who did not eat but got gastroenteritis.