Remove any dirt from mushroom caps with a damp towel and place in a 9X13, tops down.
Pour oil and balsamic vinegar over caps and allow to marinate for 1 hour.
Flip caps over so the tops are up and bake caps in over for 8 - 10 minutes, (if there is any excess moisture, it will drain out of the mushroom when baking instead of pooling inside, causing the mushroom to become soggy).
Cut mozzarella balls in half and place in a bowl. Add pimento or sun-dried tomato, olives, pesto, salt, and pepper, and stir. (For increased mushroom flavor, finely chop and saute the mushroom stems in olive oil and add to filling mixture.).
Fill each cap with the mixture and drizzle with marinade. Bake caps for 8 - 10 minutes more, or until most of the cheese is melted.