Christensad is probably Kristiansand. The Norwegian origin of Nels Nelsen is Nils (or Niels) Nilsen (Nielsen). There was no Nils born 31 of July 1886 in Kristiansand, according to online church books, so some of the information is not accurate. Then it gets difficult, because Nils Nilsen was a very common name in Norway. - And may be Nilsen in USA was his new family surname (= his father's surname), may be he is listed in 1900 census Norway using his traditional patronymic surname (Olsen if his father was Ole Nilsen). Patronymic surnames were commonly in use until 1900. May be he had two given names in Norway. That was not common in USA, so many Norwegians omitted a second given name in USA.
If you can find the wedding in USA, there is perhaps more information.