Heat olive oil in a large skillet over medium heat.
Add chopped onion and sauté until translucent.
Stir in minced garlic and cook for an additional minute.
Add Italian sausage and cook until browned on all sides.
Remove sausage and slice into pieces.
Return sausage to the skillet.
Pour in crushed tomatoes and stir well.
Add oregano, red pepper flakes, salt, and pepper.
Simmer the mixture for 20 minutes, stirring occasionally.
Serve over cooked pasta and garnish with fresh basil.