1. In a large resealable bag, mix together the orange juice, soy sauce, rice vinegar, brown sugar, ginger, onion powder, and garlic to make a marinade.
2. Place the pork chops in the marinade. Seal the bag and chill in the refrigerator for at least 2 hours and up to overnight. If possible, turn the bag every hour or so to evenly cover the pork chops with the marinade.
3. Transfer the pork chops to a shallow dish. Pour the marinade into a small saucepan.
4. In a small bowl, dissolve the cornstarch in the cold water to make a slurry. Place the saucepan of marinade over medium heat and bring to a boil. Reduce the heat to low and whisk in the slurry. Cook, whisking, until the sauce thickens, about 2 minutes. Remove from the heat and set aside to use as a baste for the grilled pork chops and pineapple.
5. Preheat an indoor grill, grill pan, or outdoor grill to medium-high heat.
6. Place the pork chops on the grill and cook until the internal temperature reaches 145°F, 4 to 5 minutes per side, brushing often with the baste as you grill.
7. Place the pineapple slices on the grill and cook for 1 to 2 minutes on each side.
8. Let the pork chops rest for 2 to 4 minutes before serving.
9. To serve, place one grilled pineapple slice on top of each pork chop.