Heat olive oil in a 12-inch heavy-bottomed skillet over medium-high heat.
Add the sausage rounds to the skillet in a single layer; sear for 3 minutes per side until browned, then remove to a side plate.
Add the diced onion and bell peppers to the residual fat in the skillet; sauté for 5 minutes, stirring occasionally, until vegetables are tender-crisp.
Incorporate the zucchini slices and continue cooking for 4 minutes until the zucchini begins to soften.
Add the minced garlic, dried oregano, and red pepper flakes; sauté for 60 seconds until aromatic.
Return the browned sausage to the skillet and toss to combine all ingredients; heat through for 2 minutes.
Season with salt and black pepper to taste; garnish with fresh parsley and serve immediately.