Glass doors turning cloudy or opaque is caused by:
Etching from acidic foods and oven cleaners. Use mild alkaline cleaners to restore transparency.
The inner and outer glass temperature difference causes moisture to build up. Keep the outer glass clean.
Hard water stains. Remove the stains with a vinegar solution and polish the glass with a ceramic hob cleaner.
Normal wear and tear over time. Heat and age can permanently affect the transparency of the oven door glass.