In a bowl, combine the steak slices with soy sauce and cornstarch. Toss to coat and set aside for 10 minutes.
Heat oil in a large skillet or wok over medium-high heat.
Add the beef and stir-fry until browned and just cooked through, about 4-5 minutes. Remove from pan and set aside.
In the same pan, add a touch more oil if needed. Stir-fry bell peppers and onion until slightly tender but still crisp.
In a small bowl, mix oyster sauce, rice vinegar, water, and sugar. Pour into the pan with the veggies.
Return the beef to the pan and stir to coat everything in the sauce. Cook another 2 minutes until everything is well combined and heated through.
Serve hot over steamed rice.