Whisk vegetable oil, soy sauce, vinegar, honey, garlic powder, ginger, and green onion together in a bowl; pour into a resealable plastic bag. Add flank steak, coat with marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator at least 4 hours.

Preheat an outdoor grill for medium-high heat, and lightly oil the grate.

Remove flank steak from marinade and shake off excess. Pour marinade into a small saucepan and set aside.

Cook flank steak on the preheated grill until firm and reddish-pink and juicy in the center, about 5 minutes per side. An instant-read thermometer inserted into the center should read 130 degrees F (54 degrees C) for medium-rare. Remove from the grill and cover with several sheets of aluminum foil. Let rest for 10 minutes, then slice thinly against the grain.

While the steak is cooking and resting, bring marinade to a boil over medium-high heat. Reduce the heat to medium-low and simmer for 10 minutes. Pour over sliced steak to serve.