Heat oil in large skillet over medium-high heat.
Add onion and garlic; sauté until onion is very tender, about 8 minutes.
Add beef and sauté until brown, breaking up with back of fork, about 5 minutes. Add chili powder, oregano, and sugar; stir 30 seconds.
Add beans, corn, and tomato sauce. Cover; simmer until beef is cooked through, stirring occasionally, about 8 minutes. Mix in cilantro and lemon juice. Season with salt and pepper.
Line 8 servings bowls with tortilla chips. Spoon beef mixture over chips. Top with cheese and sour cream.