Fresh olives are very bitter and must be 'cured' or pickled in brine to render them edible, so they are naturally long-keeping. However, when removed from their brine they will begin to go soft and deteriorate in flavor. Once the jar or can has been opened, or if bought loose in their brine, olives should be stored in the refrigerator and eaten within a few days. Those stored in olive or some other cooking oil will keep for about two weeks in the refrigerator, as long as they are completely submerged in the oil. The oil will take on some flavor from the olives and this delicately flavored oil is very pleasant for cooking.
** Asian Recipes