How can you open the oven door to clean the inside of the glass?

Glass oven doors are commonly double-glazed to keep the efficiency of the oven. Otherwise, the heat would leak out. These are sealed units, and there should be no need to clean the inside of the double-glazed unit.

As for char etc baked on to the inner surface of the unit, normal domestic cleaners should prove adequate.

But beware of any treatment that would scour or mark the glass surface. As this could cause failure of the glass, which is usually tempered glass.