Gently wash the mushrooms and remove the tips of the stems with a knife. Cut larger mushrooms into halves or quarters.
Bring a medium pot of water to a boil. Add the mushrooms and boil them for 5 minutes until just tender. Drain well. Pat dry with a cloth or paper towel if needed.
Meanwhile, combine the olive oil, vinegar, garlic, shallot, parsley, oregano, marjoram, salt, and pepper in a large bowl and whisk well to combine.
Add the mushrooms to the bowl while they are still warm and stir well to coat them in the marinade. Taste and adjust seasonings if desired.
Transfer the mixture to an airtight container and let it marinate in the refrigerator for at least 8 hours and up to 5 days, stirring occasionally.
Bring marinated mushrooms to room temperature before serving.