Place water in a large saucepan and bring to a boil.
Trim the mushrooms by cutting off a thin slice of the stem of each mushroom. Drop the mushrooms into boiling water and reduce the heat to low. Simmer for 5 minutes; do not boil. Drain the mushrooms and cover with cold water. Let stand until cool. Drain and pat dry with paper towels.
In a large bowl, whisk together the olive oil, vinegar, garlic, salt, oregano and red pepper flakes. Add the mushrooms to the olive oil mixture and stir with a rubber spatula until well coated. Let stand at room temperature for 2 hours or longer, stirring occasionally (overnight is even better). TIP: You can also marinate the mushrooms in a large zip bag. I prefer to do it this way since it's so easy to turn the bag and redistribute the marinade.
When ready to serve, drain the mushrooms from the marinade (reserving a little bit of the marinade) and place the mushrooms on a platter with toothpicks for serving. Drizzle some of the reserved marinade on top