Which pair of the animal groups, has oxygenated and de-oxygenated blood in the heart separately?

The four-chambered heart is present in birds (class Aves) and mammals (class Mammalia) where the venous blood is completely separated from the arterial blood. 

The blood circulates twice through the heart in a process known as double circulation.

In this method, the oxygenated blood received from the lungs is pumped into the arterial system which circulates it throughout the body. The venous system collects the deoxygenated blood from the rest of the body and pours it back into the heart which is then sent to the lungs for oxygenation.

This process is highly efficient as the oxygenated and deoxygenated blood does not get a chance to mix.