Heat the oil and butter in a large skillet over medium-high heat. When the butter has melted, add the mushrooms, onion, tomatoes and olives; cook, stirring occasionally, for 5 to 6 minutes, until the mushrooms have softened and released some moisture. Remove from the heat; add the cheese and dried oregano, stirring to mix well.
Place each tenderloin between 2 sheets of heavy-duty plastic wrap. Pound with a meat mallet or a small, heavy skillet to a thickness of about 1/4 inch. Season both sides of each one with salt, if desired, and pepper to taste.
Transfer one pounded tenderloin to the roasting pan. Spread about half of the filling on top, leaving a 1/4-inch border around the edges. Then roll up the tenderloin jellyroll-style, keeping the filling in place by positioning the completed roll seam side down.
Repeat with the remaining tenderloin and most of the remaining filling, arranging the second rolled tenderloin parallel to the first one and spaced about 2 inches apart. Scatter the remaining vegetable-cheese mixture around the tenderloins in the pan.
Roast for 15 minutes, then reduce the temperature to 400 degrees and roast for 15 to 20 minutes, or until a meat thermometer inserted just into the pork registers 160 degrees.
Let cool for 5 minutes before cutting crosswise into thick slices.