>>The plural of CMS is CMSs >>The plural for OER is OER as the 's' is embedded in OER ("Open Educational Resources").
I wonder if that might be confusing, making OER an exception.
>>The OERs' licenses would need to be compatible.
If the "s" is embedded, shouldn't it be OER' (though that seems strange).
Yes - strange as it may seem :-). So I would go with OER' for plural possessives.