Well, I certainly was not implying that they were.
Although to take up your point, the same can be said of many substances, whether salts, sugars, metals, calcium or any other chemical found in the body. Salt on my celery is a food, saline whether administered via IV or orally is a drug. Too much or not enough in my body will kill me, although not as quickly as a lack of O2.
I will also point out though, that under many circumstances oxygen is a deadly poison. Long term breathing of 100% O2 is potentially very harmful, and extreme depth divers will breathe mixtures containing around only 4% O2.