Christian Olson

This family tree suggests that Christian Olson's parents were Ole Larsen and Gjertrud Johansdatter:
https://www.familysearch.org/tree/person/sources/LQYV-QD8
(There are birth records and census records in there from Denmark, and the name and birth year match for this Christian Olson, but nothing conclusively proves that it is the same person since his name is fairly common)

Update: this is likely correct because the month and year of the birth date (May 1841) match his birth date as recorded in the 1900 census. Also, there is a recorded move away from his hometown (Tingjellinge, Sorø, Denmark) on Nov. 3, 1861, five years before he arrived in the U.S. However, I have not been able to conclusively rule out another Christian Olson from Sollerod, Copenhagen, who was also born in May 1841:
https://www.familysearch.org/tree/person/details/MQGS-8C8

UPDATE 07/2024: a family document (Howard W Gransee Family History) lists his birth date as May 10, 1841, which is another data point supporting this identification for Christian's birth record / parents. Another piece of evidence confirming this match is that Christian's parents names were used for two of Christian's children.

Denmark Records

1841 Birth:
https://familysearch.org/ark:/61903/1:1:QG8L-WQJR

1861 Move away from Tingjellinge, Sorø, Denmark:
https://www.familysearch.org/ark:/61903/1:1:QG85-D5VG

U.S. Records

1870 census (before the wedding. a farm laborer living in the household of Hans Boyson, who was probably his employer):
https://www.familysearch.org/ark:/61903/1:1:MD41-BDQ

This same Hans Boysen's autobiography can be found here:
http://oiseth.org/Ole_and_Lena:_genealogy/?page_id=163

1870 Marriage record:
https://familysearch.org/ark:/61903/1:1:FDFQ-F92
https://www.familysearch.org/ark:/61903/3:1:3QS7-L9XF-YVLG?i=243

1875 Census showing children Lars and Gertrude, with the family living near Serena's parents Mads and Anne Marie (spelled Anne Mary):
https://www.familysearch.org/ark:/61903/1:1:MKNZ-DF1

1880 census (before Serena's death):
https://www.familysearch.org/ark:/61903/1:1:MZ93-PR5

1882 Marriage record to Margaret:
https://www.familysearch.org/ark:/61903/1:1:Q2M5-4W22

1885 census showing Christian and his children (Serena and Margaret had both died by this point):
https://www.familysearch.org/ark:/61903/1:1:MQXC-Z27

1895 census says he was born in Denmark and that he had been a resident of Minnesota for 27 years (i.e. since c. 1868).

1900 census says he was born in May of 1841, and that he immigrated in 1866.

1909 - Grave:
https://www.findagrave.com/memorial/31960831/christian-olson

This genealogy website for Dodge County, MN shows his birth date as May 10, 1841:
http://oiseth.org/getperson.php?personID=I4622&tree=dodgo