Too much oxygen can be bad for MG, too. If you can't expel it, for example. That's why the best place is in the hospital, so they can run an arterial blood gas.
NO, it shouldn't be so damn hard to get a measly phone call. Perhaps you can ask the receptionist if the doctor would like you to die on their watch?
With an MG diagnosis in place, that makes this care extremely stupid and dangerous. Don't wait, go in.

Annie