Because built-in ovens don’t have adjustable legs, their level depends on the cavity. Therefore, before installing your built-in oven use a spirit level to ensure the cavity is level. To do this, place the spirit level along the width, depth, and diagonally across the bottom of the housing. This will give you a good reading of how level the cabinet is. If you find it’s not level, you can fix it by installing wooden boards or something similar. If you already have a built-in oven that you are replacing and haven’t experienced any problems previously, you should be good to go. Nonetheless, it doesn’t hurt to double-check.