In 10-inch skillet, melt butter over medium-high heat. Add onion, bell pepper and garlic; cook and stir 2 minutes. Add mushrooms, oregano and salt; cook and stir 2 to 4 minutes or until onion and bell pepper are tender.
With slotted spoon, top each tortilla with mushroom mixture. Sprinkle each with cheese. Roll up tightly; secure with toothpick.
Wipe skillet clean with paper towel. Add oil to same skillet; heat over medium heat until hot. Add flautas; cook 4 to 6 minutes, turning occasionally, until filling is hot and tortillas are toasted. Remove toothpicks before serving. Serve with sour cream and guacamole.