When reheating rhubarb oatmeal bars, it is best to use a microwave or oven to maintain their texture. If using a microwave, heat them in short intervals to avoid drying them out. For the oven, preheat it to a low temperature (around 300°F or 150°C) and warm the bars for a few minutes until heated through. Reheating them in the oven may help restore some of the crispness to the oatmeal crust and topping. Remember not to overheat them to prevent excessive drying or burning.