Prep squid: Clean and cut squid into rings or strips. Pat dry.
Prep vegetables: Slice bell peppers, carrots, onion, and mince the garlic.
Heat oil: Heat vegetable oil in a wok or large pan over high heat.
Stir-fry squid: Add squid and stir-fry for 2–3 minutes until just cooked. Remove and set aside.
Cook veggies: In the same pan, add garlic, onion, carrot, and bell peppers. Stir-fry for 3–4 minutes until slightly tender.
Combine and season: Return squid to the pan. Add soy sauce, oyster sauce, sesame oil, salt, and pepper. Stir well.
Thicken sauce (optional): Mix in cornstarch slurry and cook for 1 minute until the sauce thickens.
Serve: Serve hot with steamed rice.