Drain the olives and discard the brine. Using the back of a spoon, press each of the olives just hard enough to crack it open. Do not remove the olives' pits.
Put the olives in a small bowl and add fennel seed, peel, tarragon, salt, and oil. Turn the olives to coat them. Cover the bowl and let the olives marinate in the refrigerator for a week before serving. Stored in the refrigerator, they will keep for a month or longer.