Birth Information

Hi, I am wanting to find my gt grandparents. I have the following information on my grandparents from my fathers naturalisation papers when he came to NZ. His mother was Karoline Marie Olsen b 12th Oct 1873 in Bergen His Father Kristian Martinius Svendsen b 26th June 1875 in Oslo. I already have information from the 1890 Census however would like to be able to find their parents and am not sure where to write to to find out. I have my fathers birth/confirmation certificate which I had translated and he was christenedin Oslo and confirmed in Petrus. I presume Petrus is a suburb of Oslo. Could someone advise me where to go to/or look for a birth certificate for my grandparents Thankyou Kaaren Beverley( nee Svendsen)