Rinse the olives thoroughly under cold running water. Crack them with a hammer or the side of a heavy chefs knife (be careful not to crack the pit). In a large glass jar, place a single layer of olives and sprinkle them with coriander seeds and oregano. Scatter a few slices of garlic on top and make another layer in the same way. Continue until you have used all the olives and spices, then add enough olive oil so that the olives are completely covered. Leave in a dark place for 2 to 3 weeks.
Drain and serve as needed and, after the olives have been consumed, use the oil for a vinaigrette.