For the sauce:
In skillet, heat oil over medium heat; add onion, mushrooms, pepperoni, green pepper and oregano, stirring often, for 5 minutes or until onion is softened. Add tomato sauce, salt and pepper; cook for 3 minutes or until gently boiling.
Meanwhile, in large pot of boiling salted water, cook spaghetti for 8-10 minutes or until tender but firm. Remove a strand or 2 and taste it. If it's crunchy, its under done. Don't throw it up against the wall to see if it sticks. That's a myth and makes the wall hard to clean.
Drain well and return to pot. Add sauce; toss to coat well. Serve on warmed dishes with your favourite side salad.