Bertha Jensen

Hi... am trying to locate/confirm family of my husband's Great Grandmother, Bertha Jensen. From her marriage record to Peter (Niles) Petersen in Chelsea, Massachusetts 1889, it states her parents were Nikoli & Helene. From her death record in West Boylston, Massachusetts, 1951, it states she was born 7 October 1858 in Oslow. She immigrated to the U.S. some time in the mid 1880's. So she should, in theory, be on the census records for Norway. However, I have searched the census records but can not find a family which matches up with the names. Not sure... am I missing something or is there a trick to using the census data? I'm not versed in Norwegian so it's entirely possible I'm just not doing the searches correctly. ;( Is there a kind soul who might be able to shed some light on Bertha and her family perhaps? Andy help or advise would be very much appreciated. Thank you, Linda