Place the shrimp in a medium bowl. Toss with 2 teaspoon olive oil, ¼ teaspoon salt and ¼ teaspoon pepper.
Heat a large nonstick or ceramic skillet over medium-high heat. Add the shrimp in a single layer and about 1 minute per side, flipping over the shrimp with tongs. If the shrimp don’t fit into the skillet in a single layer, cook them into 2 batches. Transfer the shrimp to a plate or bowl.
If there are a lot of browned bits stuck to the bottom of the skillet, either carefully (pan is hot!) wipe them out with a damp paper towel or wash the skillet.
Reduce the heat to medium. Heat the remaining 2 teaspoons olive oil in the skillet. Add the onion and cook, stirring occasionally, until the onion softens and becomes translucent, 4 to 5 minutes.
Stir in the garlic, oregano, ½ teaspoon salt, ¼ teaspoon pepper and crushed red pepper flakes. Cook, stirring, for 1 minute.
Add the crushed tomatoes. Cook at a gentle simmer, stirring occasionally, for 15 minutes.
Stir in the cooked shrimp and parsley. Taste and adjust seasoning, if necessary. Serve with pasta, rice, quinoa, cauliflower rice, spaghetti squash or zucchini noodles.