Hello, and welcome to the site
There are pretty clear cut rules for a diagnosis of MS. You need dissemination in time (i.e. at least two relapses/attacks/episodes) OR gradual progression over at least a year (or that is sure to happen) and dissemination in space (i.e. at least two areas of the central nervous system affected). Your ON will very probably count as one attack (more likely if it was diagnosed by a medical professional and properly documented). Whether or not your current symptoms count as a second is impossible to say as you don’t give details, but even if they do, MS is not the only cause of ON or of neurological symptoms so it is still possible that something else is behind your symptoms so you should try and keep an open mind.
I suggest that you google Polman et al (2011). Diagnostic Criteria for Multiple Sclerosis: 2010 Revisions to the McDonald Criteria. ANN NEUROL 2011;69:292–302. This paper details the latest diagnostic criteria and should explain how your ON, as one attack, fits in with this.
To more directly answer your question, whether or not these latest symptoms qualify you for a diagnosis of MS depends on two things: if the neuro accepts the ON as a first attack and this latest batch of symptoms as the second, and how many lesions you have on MRI, as well as where they are. If the neuro accepts the ON and the current symptoms as two attacks, then you are relying on MRI / lesions for a diagnosis. If there is at least one lesion in two different MS-typical areas, then nothing else is needed. If there is only one in an MS-typical area, then you will have to wait for another lesion in a typical area (according to the rules). It is normal for a second scan to be a least a month later, but there is no set rule.
The only time a second MRI comes into play is either to show a new lesion (dissemination in time) or to show a second area of the nervous system (dissemination in space). So, if your MRI shows multiple lesions, satisfying the McDonald criteria, then there is no need for a second.
As far as lesions at the onset of ON go, no, I would not describe them as common. Basically, 50% of people with ON go on to develop MS. Many of these will have no lesions when they first present with ON. IF they have lesions, they are more likely to develop MS, but even then it is not guaranteed.
As far as ON = CIS goes, then yes, it does. CIS is basically part dissemination in time (i.e. one attack) and part dissemination in space (i.e. one part affected). Neuros are funny creatures though - they may not accept a diagnosis of something they haven’t witnessed personally: so your ON is more unlikely to be accepted as a CIS unless it has been properly documented by an ophthalmologist.