Yes, you can certainly prepare Okonomiyaki ahead of time. Once cool, keep the pancake in a sealed container in the refrigerator for up to 5 days. Reheat by cooking quickly in a hot skillet with a bit of oil until warm. Or, place on a lined baking sheet and bake at 350 degrees Fahrenheit for 10-15 minutes. Make the sauce up to 2-3 days before you plan on using it, and keep it stored in a container with a lid in the fridge for up to a week.