Heat a grill to high (450° to 550°). Mix oyster sauce, oil, pepper, and garlic in a large bowl. Brush 3 tbsp. oyster sauce mixture over pork and 2 tbsp. over onion. Set remaining sauce mixture aside.
Oil cooking grate with a wad of paper towels, using tongs. Grill onion and pork, turning once, until grill marks appear, 6 to 8 minutes total for onion and 8 to 10 minutes total for pork. Let rest about 5 minutes.
Meanwhile, stir vinegar and sugar into remaining oyster sauce mixture. Add spinach, sprouts, and cucumber, tossing to coat.
Divide salad among plates. Thinly slice pork across the grain and arrange over salads. Separate onion slices into rings and lay on pork slices. Drizzle pork with any remaining juices and dressing.
*Find in the Asian foods aisle.