I was being treated with nitrous medically. I asked the anaesthesiologist about how it works recreationally and his answer was that yes, it was mostly just hypoxia.

This is easily falsified by a cursory internet search about the physiological mechanism behind nitrous oxide's effects. It is appalling that a medical professional would so confidently give you an uneducated, crackpot answer. The exact same mechanism which knocks you out gives you euphoria at lower doses.

If someone holds their breath long enough to cause hypoxia when inhaling nitrous oxide, they have other problems. You can easily hold your breath 1-2 minutes while sitting on a couch without experiencing hypoxia. If you're experiencing euphoria as strong as what nitrous oxide causes from hypoxia, you're basically about to die.

This is why you shouldn't trust experts on stuff outside their speciality, this answer is just wrong.

You don't even need to research it, the lived experience of being in a dentist office with mixed oxygen and nitrous produces the recreational effects - if it was mostly hypoxia, having oxygen mixed in would have a greatly diminished "recreational" effect.

I mean, it is true most people doing it recreationally are giving themselves mild to severe hypoxia, but that doesn't mean the effect is caused by hypoxia

Bullshit. You aren't supposed to be hypoxic when using nitrous. You take a half-breath of air, and then breathe in the n2o.