Some Shivlings are oval shaped especially the Banalinga. God as per Hindu concept is formless and That can only be represented symbolically. Shivling in the shape of oval is one such form.
Symbolically the oval-shaped Shivling represents the universe. It is divided into two halves sky and earth.
The oval has the shape of an egg. Egg is considered the source of life in which both male and female forms are united.