What prevents oxygenated blood from mixing with deoxygenated blood?

The septum.

In the heart, the septum is a muscular wall that separates the left and right sides. The left side of the heart pumps oxygenated blood to the body, while the right side pumps deoxygenated blood to the lungs. The septum prevents the two types of blood from mixing, which would interfere with the body's ability to circulate oxygen-rich blood to its tissues.