Many pulleys need an oval-shaped carabiner to work properly. The OK carabiner is designed for just that. It is also very useful at belays.
The oval OWALL is a general purpose carabiner for trad and aid climbing. Ideal for racking wired nuts and other protection: gear does not snag on the Keylock.
The symmetrical carabiner was the first option in connectors and also today its smooth and symmetrical shape is a beloved choice for simplicity and manoeuvrability.