I think it’s framed in the context of: “How dangerous would a single molecule be to a human?”. In that context, I would say O is safe, only because our body naturally destroys the radical oxygen molecules every day that we create with our anti-oxidants.

True, in a larger quantity than our body can handle, it’s extremely toxic; but a single molecule would probably not be too bad.

But I do agree, it shouldn’t be Green. It should be Yellow at least.