Preheat oven to 350 degrees F. In a large saucepan cook potatoes in enough lightly salted boiling water to cover for 15 to 20 minutes or until tender; drain. Mash with a potato masher or beat with an electric mixer on low speed. Gradually add sour cream, milk and salt, mashing or beating to make potato mixture light and fluffy. Stir in 1/2 cup cheese.
Meanwhile, in a large skillet cook pork, sweet pepper, onion, and garlic over medium heat until brown, stirring to break up meat as it cooks. Drain off any fat. Stir in undrained tomatoes, barbecue sauce, corn, pickle, mustard, and oregano. Bring to boiling; reduce heat. Simmer, uncovered, for 5 minutes to blend flavors.
Spoon pork mixture into a 2 1/2-quart baking dish. Spoon mashed potatoes in mounds over pork mixture. Sprinkle with remaining 1/4 cup cheese. Bake about 20 minutes or until heated through and cheese melts. Top with green onions and bacon, if desired.