I believe the brain is too complex to compare to either an operating system or a CPU. We can perhaps analogize both of those artificial entities to individual functions that a human brain may perform, but they would not be comprehensive of its abilities. As just one additional ability, there's also a memory storage (both short-term and long-term) mechanism existent in the brain, perhaps akin to temporary and permanent memory in a computer. Artificial neural networks mimic the properties of biological neurons. And then there's the concept of intelligence and actually thinking and understanding (which is really much more complicated than mere processing as in terms of a CPU); while machines' abilities are more along the lines of imitation/simulation.