There's no pressurised oxygen, only hydrogen, the oxygen is extracted from ambient air as needed. If you work out a safety valve system that's good enough (whic

There's no pressurised oxygen, only hydrogen, the oxygen is extracted from ambient air as needed. If you work out a safety valve system that's good enough (which has presumably been done, since Toyota Mirais are driving around where I live), rapid release should be reasonably safe. Hydrogen is much lighter than air and will evaporate quickly. It's not as safe as diesel, but no worse than gasoline, or even large Lithium batteries.