Here is the wedding of Lise Mathilde (Elise Mathilde) Grøn and Isak Olsen (couple nr 13, upper left side) : Permanent sidelenke:
http://www.arkivverket.no/URN:kb_read?i ... _side=-320
Her father`s name is written Olaus Nikolai Grøn, and his father`s name is Ole Knudsen.
You call him Isak Knudsen, but as you can see, he had the name Olsen because his father`s first name was Ole
As this is not my family and I know nothing about them, I can just say that if the father of Olaus Nikolai Grøn had Niels as first name, then Olaus also could be called Olaus Nikolai Nielsen Grønn. This to explain why a name like Nielsen could appear...or you have wrong information from somewhere, and he didn`t have the name Nielsen at all

. You have to find the birth of Olaus Nikolai to see his father`s name.
Regards from
Linda