Could we say: Mama mea e o chimista? And should we add "o" or can say like in the original example? And why in the orignal is no "o"?
Mama mea e o chimistă is a perfectly valid sentence but it has another meaning, that of My mother is a chemist which is not the same as The chemist is my mother.
The way I see the later is as if there were some people, like a doctor, a chemist and a farmer and you'd say that the one who's the chemist is your mother. Sure, you can also say that your mother is a chemist but it won't mean that she is that chemist. Reason for which, to convey the same meaning, you'd have to say Chimista e mama mea.
OK, understood. And what about "o"? Can we omit it while talking in a context like above?
Indeed. Actually it is more usual to say Mama mea e chimistă instead of o chimistă.
Oh, and if you meant to say Mama mea e chimista instead of chimistă then it is pretty much the same as Chimista e mama mea, but the initial o confused me a bit.