Is all death lack of oxygen? is all brain death lack of oxygen? can humans die solely from too much oxygen?

Too much Oxygen: Nope. But too much oxygen can burn tissue and cause damage that can be permanent.

Not for "BrainDeath": Yes lack of oxygen is a cause of brain damage from a variety of causes. One can also have brain death from trauma (head injury). Also metabolic issues such as to low blood sugars. Also too much oxygen can damage the lung tissue.