To save last-minute prep time -- and allow the flavors to develop even more -- make these tangy mushrooms a day ahead. Serve them with store-bought olives and pickled hot or sweet peppers as an antipasto course.
Trim stems from cremini (break apart oyster mushroom clusters, if using). Put mushrooms in a medium bowl, and set aside.
Whisk together lemon juice, vinegar, and oregano in a medium bowl. Whisk in oil in a slow, steady stream; whisk until mixture is emulsified. Whisk in the salt and pepper. Pour marinade over mushrooms, and let stand at room temperature, stirring occasionally, at least 20 minutes; if desired, refrigerate mushrooms in an airtight container overnight. Serve at room temperature.