Carefully wipe down the skillet and immediately add the remaining oil to the skillet and then the onions, garlic, and red pepper flakes. Cook, stirring, until onions are translucent, about 6 minutes. Add the tomatoes and season with oregano, salt, pepper, and stir well. Simmer until sauce thickens slightly, about 10minutes. Add the shrimp and any juices and cook for another minute for the flavors to meld. Taste and season with salt, if needed. Serve with the zucchini noodles.