Olive Mushroom Salad

8 ounces sliced mushrooms

8-ounce jar salad olives

1 can black olives, drained

4 celery stalks, chopped

2 medium onions, sliced

1 tablespoon oregano or less

1/3 cup sugar

1/2 cup plus 1 tablespoon cider vinegar

1/2 cup plus 2 tablespoons oil