Dying from a lack of oxygen is usually very painless and an easy way for someone to die.
When you lack oxygen to the brain the brain cells begin to die and without those brain cells you can't really feel pain so once the brain cells die off due to lack of oxygen the complete death from the lack of oxygen to the brain usually puts the person completely out without pain.
Although there might be a bit of pain but it's not that painful according to doctors for someone to die due to lack of oxygen.