Press saute mode. Add oil and heat up. Add garlic and chopped onion and saute until the onion is almost translucent. Add Italian seasoning and dried oregano, stir well.
Place Italian sausage (casings removed) in and cook, breaking it down with a spoon. Cook until no longer pink and there are brown bits on the bottom of the pot. Press "cancel/off" button. Add ¼ cup of water to sausage and stir well, scraping the browned bits from the bottom.
Add ziti and 2 cups of water and stir well.
Pour diced tomatoes (with juices) over the pasta, add tomato paste and crushed tomatoes. Do not stir.
Close the lid and switch the valve to sealing position.
Press "manual" and adjust timer to 6 minutes.
When the timer is done, press "off/cancel" button and quick release the pressure.
Open the lid and add mozzarella cheese and ricotta cheese and stir well.
Serve ziti garnished with fresh basil and grated Parmesan cheese.