Perhaps you are talking about a different Otto Hamann than the one below. This one does appear in Unity, Clark, Wisconsin. You give no date or place of birth on yours, so hard to know. Hope this helps.
Unity, Clark, Wisconsin
Otto H Hamann 62 born about 1878 in South Dakota, farm
Elma Hamann 39 born about 1901 in Wisconsin
Ethel Hamann 15 born in North Dakota
Elwin Hamann 14 born in Wisconsin
1920 census Otto is living with his parents in Chaseley, Wells, North Dakota
Antelope, Wells, North Dakota
D F Hamann 66 born about 1844
Mary C Hamann 62
Otto Hamann 32 born about 1878 in South Dakota
George Hamann 20
1900 census this family lived in Yankton, South Dakota.
Family Tree online says, "Otto Henry Hamann was born 1878 in Yankston, South Dakota, and died March 15, 1957 in Loyal, WI. He married Elma Lurinda Hutchins on June 29, 1921, in Township of Unity Loyal, WI. on home farm, daughter of Warren Edwin Hutchins and Clara H. Smith...1957, Loyal City Cemetery."
About D. F. Hamann - possible match in Wells, North Dakota, in 1920 transcribed as Delleff F. Hamam and in 1900 transcription is Datlas F Hamarin. I suggest the name may be Detleff (a name used in Germany and Scandinavia and...)
Searching on Detleff F. Hamann, up comes a family tree. Below is the information as reported on that tree. Detleff is said to be born in Denmark or Germany. Usually this means the person came from Schleswig-Holstein area of southern Denmark/northern Germany, a territory that changed between Denmark and Germany:
"Detleff F. Hamann. Born March 1844, died after February 1920. Detleff F. Hamann was born in March 1844 at Denmark or Germany. He married Mary C. (?) circa 1866. Detleff F. Hamann died after February 1920. Detleff F. Hamann was also known as D. F. He was listed as the head of a family on the 1900 Census at Township 95, Yankton, South Dakota.1 He was listed as the head of a family on the 1910 Census at Antelope, Wells, North Dakota.2 He was listed as the head of a family on the 1920 Census at Chaseley, Wells, North Dakota; living between John P. and Fred J. Hamann; also living with them are granddaughters Viola M. White, age 12, and Pearl D. White, age 10.
John P. Hamann b. c 1871
Fred J. Hamann b. c 1874
Otto H. Hamann b. Jan 1878
Clara A. Hamann b. Mar 1887
George K. Hamann b. May 1889
In 1900 census Mary Hamann is reported as having had 8 children, with 6 living.http://www.frielfoto.com/genealogy/p312.htm#i14197