Whisk orange juice, zest, soy sauce, brown sugar, vinegar, garlic, ginger, red pepper flakes, and sesame oil.
Pat chicken dry. Season both sides with salt and black pepper.
Heat oil in a large skillet over medium-high heat until shimmering.
Sear chicken thighs until golden on both sides, about 6–8 minutes total.
Pour in the orange sauce. Reduce heat to medium and simmer 6–8 minutes.
Stir cornstarch with water. Drizzle slurry into skillet while stirring.
Simmer until sauce is glossy and thick and chicken is cooked through.
Fold in green onions. Sprinkle with sesame seeds. Rest 2 minutes before serving.