Rinse off mushroom of any dirt, then place them in a pot and cover them with water and add a teaspoon of salt, bring it to a boil, and cook for a few minutes until the mushrooms are tender, or alternatively you can saute them in a frying pan with a little olive oil or water until tender.
Stir into the drained mushrooms, the oil, vinegar, onion, garlic, sugar, salt, oregano, crushed red pepper, and black pepper.
Add the mushrooms and mix everything together and refrigerate for 6 hours or until cold.
These will last in the fridge for 2 weeks as long as the mushrooms are covered in liquid.