Heat oil in a large nonstick skillet over medium-high heat. Add onion and pepper; sauté 3 minutes. Add beef, and cook until beef is browned and vegetables are tender, stirring to crumble meat.
Add corn, taco seasoning mix, tomatoes, and beans; bring to a boil, stirring occasionally. Reduce heat, and simmer, uncovered, 15 minutes or until mixture is slightly thick. Stir in cilantro.
Spoon 1/2 cup beef mixture into each taco shell; top evenly with lettuce and cheese.