Heat a large skillet on medium heat and add olive oil.
Add the bell pepper and stir. When the edges of the bell pepper start to turn a golden color (about 2 minutes), move them towards the outside edges of the pan.
To the middle of the pan, add the butter. Stir the butter until it melts.
In the center of the pan, add the mushrooms, garlic, pepperoni, onion, dried oregano, salt and pepper. Stir.
Continue to sauté and stir until the mushrooms are lightly browned (about 4 to 5 minutes).
Top with grated Parmesan cheese and garnish with parsley.
Serve right away while warm.