1. Warm oil in a large skillet over medium heat. Sauté onions until tender, about 3 minutes. Add garlic; sauté 1 minute. Add bell peppers and jalapeños; sauté until tender, about 3 minutes.
2. Transfer pepper mixture to slow cooker. Stir in beans, tomatoes, chili powder, cumin and oregano. Arrange squash on top. Cover and cook on low for 6 hours.
3. Season chili with salt and pepper. Serve with sour cream, salsa and other accompaniments, if desired.