Yes, you can reheat ackee and saltfish in the oven or microwave. Preheat oven to 375 degrees Fahrenheit. Arrange ackee and saltfish on a baking sheet lined with parchment paper. Drizzle with olive oil, vinegar, and diced onion if desired. Bake in preheated oven for 10-15 minutes, or until heated through. Serve immediately.