You have not mentioned whether you are on any of the DMTs - some of these require regular monitoring of thyroid function - but your doctor should be keeping on top of this. Underactive thyroid can contribute to weight gain, and can also impact on fatigue levels.
Also in relation to fatigue - folate deficiency can cause fatigue, but this should not be an issue if you are eating lots of leafy greens etc as part of your diet. Note though that if you are on one of the DMTs which can have flu-like side effects and you have to continuously take NSAIDs to counter these side effects, this can negatively affect your folate absorption - I found this out the hard way and I ended up with a folate deficiency because the flu-like side effects never went away for me and I had to keep taking the NSAIDs with the interferon. If you think this may be a factor and you get your folate checked it should be Red Blood Cell folate and not serum folate that is checked. RBC folate is a much better indicator of folate levels as it measures reserves, not just what's floating around in your blood serum.
Additionally, because the OMS diet is very low in sources of Vit B12 a deficiency of this can become a problem, and fatigue can be influenced by your B12 levels. If you get your B12 tested do not accept that it is "OK" if you are told that it is "normal". In Australia the bottom end of the reference range is 150pmo/L while in many other countries it is much higher and there is considerable discussion going on about what is really an acceptable level for B12 - with some recommending that the minimum level should be set at 400pmol/L. (It is actually estimated that around 20% of Australians actually have low B12 levels - even with 150pmol/L being used as the bottom measure.) Low B12 levels can cause fatigue, and deficiency of B12 can cause neurological damage similar to that of MS. If you look at the section on this site about B12 there is good information about this, and one of the actual articles cited can be accessed on this link:http://www.direct-ms.org/pdf/NutritionM ... d%20MS.pdf
If you decided to get your blood tested it is absolutely critical that folate and B12 are both checked. If a folate deficiency is found and treated while there is an underlying B12 deficiency which has not been identified, supplementation with folate allows the neurological damage that can be caused by a B12 deficiency to continue unnoticed (sometimes until the damage is irreversible). It is now also more widely recognised that B12 deficiency can exist and be causing neurological damage without what used to be required in the way of anaemic markers with red blood cells before a B12 deficiency was considered to exist.
Low magnesium levels can also be a factor in fatigue and weight gain, but the OMS diet should be OK in dietary magnesium. However, taking the high levels of Vit D3 as is recommend for people with MS can negatively impact on your magnesium levels - I actually had to stop my Vit D3 for a while until I got my magnesium intake up as calcium required for magnesium utilisation was being "gobbled up" by the Vit D3.
Note that folate, B12, and magnesium are all inextricably linked in being metabolised in your body, so it is not a good idea to just take a whole heap of just one of them - any supplementation must be balanced.
Of course it is entirely possible/likely that your weight gain is just an unwanted "fact of life", and that the fatigue is just MS related, or due to running around after your children, but sometimes there can be an underlying biological cause which may be worth getting checked out.