Preheat your oven to 350 degrees Fahrenheit. 

Place the onion rings in an oven-safe dish and cover the dish with aluminum foil.

Once the oven is preheated, you can place the dish of onion rings inside for around 10 to 15 minutes. Keep checking on them to ensure that they aren’t turning brown or getting burned at the edges. 

Don’t let the onion rings go back to room temperature once heated as heating them again may cause the food to crumble. Eat them when they’re still reasonably warm.