Hi Anant,
As likenoother.sam said Original Residence permit means your NL residence permit.
I am still working through the process of obtaining my Visa D from the Embassy of Belgium in Prague. I am not sure how long will it take, I hope its quick for you. I am having delay issue due to large number of applicants in the Embassy of Belgium in Prague. They told me it will take few weeks time once applied.
Best regards,
Prathik