Otto was the son, and third child, of John and Anna Boston Rapp.

He was the grandson of Jacob and Catharina Swartz Rapp, Henry and Elizabeth Boston.

He was the brother of Louis, Edward, (Otto), George, [sister - died as an infant], [Johann - died as a child], [Carl - died as a child], Elizabeth, Mary, Fred, William, and Henry.

He married Cora E. Weber on February 27, 1913 in Pontiac.

He was the father of Carl Rapp (married to Ethel Wurmnest) Rapp and Viola Rapp Cooper (married to Charles Cooper).
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Source: Pantagraph – June 28, 1934

PONTIAC – Funeral services for Otto C. Rapp, 49, Rooks Creek farmer who died at 10 p.m. Tuesday in St. James' Hospital from heart disease, were held Thursday. Rev. E. A. Kreppert, pastor of the First Lutheran Church conducted the services at the church at 2:30 p.m.

Mr. Rapp had been ill for five years and was at St. James' Hospital for several weeks.

He was born in Nebraska Township on March 16, 1885 and was married to Miss Cora Weber in this city on February 27, 1913. Since then they lived on a farm near Pontiac.

Surviving are his parents, John and Anna Boston Rapp, Pontiac; his widow and two children, Carl and Viola, at home; two sisters, Mrs. Mary Holzhauer, Owego, and Mrs. Elizabeth Weber, Eppards Point; six brothers: Lewis C., Long Point; Edward, Pontiac; George of Paton, Iowa; Henry J., Rooks Creek; and Fred and William D., at home with their parents.
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1900 United States Federal Census
Birth Date: Mar 1885
Birthplace: Illinois
Home in 1900: Nebraska, Livingston, Illinois
Race: White
Relation to Head of House: Son
Father's Name: John Rapp b. Illinois
Mother's Name: Anna M Rapp b. Illinois
Household Members:
John Rapp 43 (farmer)
Anna M Rapp 54
Edward Rapp 17 (farm laborer)
Otto Rapp 15 (farm laborer)
George Rapp 14 (at school)
Elizabeth Rapp 7
Mary Rapp 5
Frederick Rapp 3
William Rapp 1

Otto Carl Rapp was confirmed at St. Petri Lutheran Church near Flanagan on Palm Sunday, 1900, along with his brother George Rapp.

1910 United States Federal Census
Home in 1910: Pontiac, Livingston, Illinois
Household Members:
John Rapp 53 (farmer)
Anna M Rapp 53
Otto Rapp 25 (farmer)
George Rapp 24 (farmer)
Fred Rapp 13
William Rapp 11
Henry Rapp 9
Elizabeth Rapp 17
Mary Rapp 14

Illinois, County Marriages, 1800-1940
Name: Otto Rapp
Gender: Male
Age: 28
Birth Date: abt 1885
Marriage Date: 27 Feb 1913
Marriage Place: Livingston, Illinois, USA
Father: John Rapp
Mother: Annie Boston
Spouse: Cora Weber
Film Number: 001401623

Livingston County, Illinois, Farmers Directory, 1917
Given Name: Otto C. Rapp
Spouse: Cora Weber
Children: Carl, Viola
Post Office: Pontiac
Section: 5
Twp Name: Pontiac
Twp #: 36
Owner/Tenant: tenant
Acreage: 160
Year Became Resident of Livingston Co: 1886
Owner of Farm (If Tenant): Matt Phipps

U.S., World War I Draft Registration Cards, 1917-1918
Date: September 10, 1918
Name: Otto Carl Rapp
Address: RR5, Pontiac
County: Livingston
State: Illinois
Birth Date: 16 Mar 1885
Race: White
Draft Board: 20
Age: 33
Occupation: Farming (self-employed)
Nearest Relative: Cora Elsie Rapp (wife)
Height/Build: Medium/Medium
Color of Eyes/Hair: brown/dark brown

1920 United States Federal Census
Name: Alto Rapp [Otto Rapp]
Home in 1920: Pontiac, Livingston, Illinois
Relation to Head of House: Head
Able to read and Write: Yes
Household Members:
Alto Rapp 34
Cora Rapp 26
Carl Rapp 6 4/12
Viola Rapp 4 9/12

1930 United States Federal Census
Home in 1930: Rooks Creek, Livingston, Illinois
Neighbors: Living nearby were Charlie E, Elsie E, Donald, and Richard Weber.
Household Members:
Otto C Rapp 45 (no occupation listed)
Cora E Rapp 36
Carl G Rapp 16 (farmer)
Viola M Rapp 14

Illinois, Deaths and Stillbirths Index, 1916-1947
Name: Otto C. Rapp
Birth Date: 16 Mar 1885
Birth Place: Nebraska Twp, Liv. Co , Illinois
Death Date: 26 Jun 1934
Death Place: Pontiac, Livingston, Illinois
Burial Date: 28 Jun 1934
Burial Place: Rooks Creek, Livingston, Illinois
Cemetery Name: Rooks Creek
Death Age: 49
Occupation: Farmer
Residence: Rooks Creek Twp, Illinois
Father Name: John Rapp
Father Birth Place: Bureau Co., Illinois
Mother Name: Anna M. Boston
Mother Birth Place: Bloomington, Illinois
Spouse Name: Cora Rapp
FHL Film Number: 1711834