Put the vegetable oil, soy sauce, rice vinegar, minced garlic, minced ginger, onion powder, and black pepper in a bowl and whisk to combine.
Flatten the chicken breasts to an even thickness and then place them in a gallon-sized storage bag. Pour the marinade over the chicken, seal the bag, and turn to coat. Place in the fridge to marinate for 30-60 minutes.
Preheat the grill to medium heat. Grease the grill grates before adding the chicken. Remove the chicken from the marinade and discard the remaining marinade. Grill the chicken for 13-16 minutes, turning it over halfway through cooking or until it reaches an internal temperature of 165˚F. During the last few minutes, brush the chicken with teriyaki sauce and flip frequently to glaze it.
Garnish with sliced green onions and sesame seeds.