Heat oil in a large saucepan over medium heat. Add onion and green bell pepper; cook until vegetables are tender, 5 to 7 minutes. Add garlic, red pepper flakes and oregano; cook until fragrant, about 1 minute. Place the mixture into a 6-quart slow cooker.
Add the chicken stock, chicken, diced tomatoes, tomato paste, Italian seasoning, salt and black pepper; turn on high and cook for 10 minutes. Reduce heat to low and cook 6 to 8 hours.
Remove chicken and shred, then return to slow cooker. Add pasta and cook on high until pasta is cooked al dente, about 10 minutes. Add cheese and stir until melted. Season again with salt and pepper to taste. Sprinkle with fresh basil and serve.
Heat oil in a large saucepan over medium heat. Add onion and green bell pepper; cook until vegetables are tender, 5 to 7 minutes Add garlic, red pepper flakes and oregano; cook until fragrant, about 1 minute.
Add the chicken stock, chicken, diced tomatoes, tomato paste, Italian seasoning, salt and black pepper; bring to a boil. Reduce heat and simmer for 10 minutes.
Remove chicken and shred, then return to the pan. Add pasta and simmer until pasta is cooked al dente, about 10 minutes. Mix in Parmesan cheese and stir until melted. Season again with salt and black pepper to taste. Sprinkle with fresh basil and serve.