Benign oven cracks
Oven cracks are usually not life threatening. I really don't know how to build an earth oven that will not develop some cracks. It is a normal phenomenon for clay to crack when it dries. The cracks in the oven shown probably amount to nothing. The greater percentage of clay to sand in your mixture means more cracking. Too little clay means that your oven will fall apart. It's happened to me.
Generally, you want 3 parts sand to 1 part clay.
Dry clay soil in garden