Place sausage links in the bottom of slow cooker.
Top with sliced peppers and onions.
Pour crushed tomatoes and dried oregano or Italian seasoning on top.
Season with salt and pepper.
Add basil leaves on top of tomatoes.
Cover and cook on low for 7-8 hours.
Serve on hoagie rolls topped with shredded mozzarella cheese or over rigatoni.
I didn't put exact measurements for salt and pepper since it really depends on what brand of crushed tomatoes you use. I tend to use no salt when possible so I use a little more salt. If you use a brand that may already have salt in it you may want to use less. What I suggest is start by adding a little, maybe 1/4 t or so and you can always add more at the end if needed.