You are right, Shaun', and your advice is right.
And at the same time . . . this doctor seems to know that charles' has CH, and, as Pebbles' keeps reminding us, virtually all doctors have an online or hardcopy resource that tells them what the evidence-based prescription should be. Oxygen will be listed right there, in uncomplicated terms, as the #1 or 1a abortive, along with sumatriptan/zolmitriptan (and amitriptyline probably won't be mentioned at all), so Pebbles' is also right that not prescribing oxygen is pretty darn close to inexcusable.