Why is moisture visible on or between the oven door glass?
This is seen most often on gas ovens.
When the gas oven burner ignites & heats, the primary combustion by-products are water (in the form of steam) and carbon dioxide. You are seeing the steam condensing on the cool surfaces of the oven door. Once the oven heats to temperature, bringing the cool surfaces to temperature, the steam will no longer condense on the door glass and the moisture will dissipate.
Factors that can increase condensation on or in the door:
High-moisture foods such as turkey, roasts, and casseroles may cause moisture to form on the inside and between the door glass of the oven door. This is temporary and the moisture will dissipate during the cooking process.