It is not dangerous to heat up an oven and then place food inside once it's hot. In fact, that is the standard procedure for cooking in an oven. Heating the oven first allows it to reach the desired temperature for even cooking.
Here's the typical process for using an oven:
Preheat the oven: Set the desired temperature on the oven and allow it to heat up fully. This usually takes 10-15 minutes, depending on the oven.
Prepare the food: While the oven is preheating, you can prepare the food you want to cook or bake.
Place the food in the oven: Once the oven reaches the desired temperature, open the oven door and carefully put the food inside using oven mitts or appropriate utensils. Avoid placing your hands or body too close to the oven's hot surfaces to prevent burns.
Close the oven door: After placing the food inside, close the oven door securely to maintain the temperature inside the oven.
Monitor cooking time: Set a timer according to the recipe or cooking guidelines for the particular food you are preparing. Keep an eye on the food through the oven door window or by briefly opening the door if necessary, but try to minimize opening the door to avoid temperature fluctuations.
It's important to note that ovens can get very hot, so exercise caution when handling hot surfaces and use appropriate protective gear like oven mitts. Additionally, follow the manufacturer's instructions for your specific oven model and refer to any safety guidelines provided.
Overall, as long as you handle the oven and hot food with care, there should be no inherent danger in heating up the oven first and then placing food inside once it's hot.