Where the Wall in Olaus B Wall come from

I'm trying to figure out why Olaus named changed to Wall. Any help would be appreciated. In the 1870 census he was in Dodgeville WI as Olavus Olson. His marriage was recorded as Olaus B. Wall Date: 27 Jul 1879 Place: Mitchell, Northwood, Iowa Spouse: Anna Johnsen. His death recorded as Olaus B. Wall Death Date: 29 Mar 1948 Death County: Kandiyohi, MN. His parents marriage Record #17 Benjamin Olsen marriage June 27 1845 Record #60 his sister's Karen's birth 2 Sep 1848 Record #6 his sister's birth/christening Gunnild Marie Dec 16 1851 Permanent pagelink: http://www.arkivverket.no/URN:kb_read?idx_kildeid=1130&idx_id=1130&uid=ny&idx_side=-22 Record 25 his birth/christening Olaus Permanent pagelink: http://www.arkivverket.no/URN:kb_read?idx_kildeid=1130&idx_id=1130&uid=ny&idx_side=-45 Record 32-36 May 30, 1855 migration til amerika Permanent pagelink: http://www.arkivverket.no/URN:kb_read?idx_kildeid=1130&idx_id=1130&uid=ny&idx_side=-341