In a large skillet over medium-high heat, add the oil and the chicken. Stir the chicken to coat it in the oil. Let the chunks of chicken cook until they start to brown on a couple of sides, stirring occasionally (about 5-7 minutes).
In the meantime, add the orange juice, soy sauce, brown sugar, water, corn starch, and black pepper in a bowl. Stir or whisk until well combined and corn starch is dissolved.
Pour the sauce over the browned chicken and stir to coat. Once sauce has come to a bubble, reduce the heat to a simmer. Let the chicken and sauce simmer for 10 minutes, or until chicken is cooked through.
Serve hot. Sprinkle with optional green onions and/or sesame seeds.
