Remove stems from mushrooms; save for another use. Place mushrooms in a self-sealing bag set in a bowl. Stir together oil, vinegar, and pepper. Pour over mushrooms. Seal and marinate in the refrigerator for at least 4 hours or up to 24 hours, turning bag occasionally.
Stir together cheese spread and mayonnaise. Fold in crabmeat and green onion.
Drain mushroom caps, discarding marinade. Pat caps dry. Place on baking sheet. Fill with crabmeat mixture. Broil 4 inches from the heat for 5 to 8 minutes or until bubbly and golden. Serve warm. Makes 36 to 40 appetizers.