I suspect (yet again) that your neurologist is a difficult person to like.
He seems to have confused and muddled his way through the diagnostic process with you - and perhaps does with all his patients, at least the non straightforward ones. And let’s face it, everyone is different, so no one is straightforward. And he does seem to have the personal skills of a non very friendly scorpion.
This is how I am reading his diagnosis / non diagnosis. You have Oligoclonal bands present in the cerebrospinal fluid. Now something like 80 to 95% of people with MS are positive for O bands. I don’t know whether you can have O bands in the CSF, but no lesions and so not have MS. Equally, your neurologist seems to be saying that because you have O bands but no lesions, you both do and don’t have MS. Very confusing.
The Oligoclonal bands show up the presence of inflammatory activity in the CSF. Normally, in MS, this results in lesions either in the brain or on the spine. Or both. (I’ve assumed he’s done an MRI of your spine as well as your head? If not, you should ask him to refer you for one.) So ordinarily, people with O bands would also have lesions and therefore be diagnosed with MS.
So that would seem to say, yes there is inflammation, but it’s not actually translated into lesions or definitive injury to the brain or spine. The McDonald Criteria https://www.mstrust.org.uk/a-z/mcdonald-criteria says that without lesions, you can’t be diagnosed with MS. Which is why you’ve not been diagnosed.
So all he can do is to diagnose FND, because clearly there is something wrong, something real, but he is just not able to diagnose MS. Which means he is not able to prescribe a disease modifying drug.
Does this make sense to you? And does it help or just confuse things even more?