View source for O'Neill, Nebraska

===2000 census===
At the [[2000 United States census|2000 census]], there were 3,733 people in 1,554 households, including 988 families, in the city. The population density was {{convert|1,580.7|PD/sqmi|PD/km2|sp=us|adj=off}}. There were 1,740 housing units at an average density of {{convert|736.8|/sqmi|/km2|sp=us|adj=off}}. The [[Race and ethnicity in the United States Census#2010 census|racial makup]] of the city was 98.53% White, 0.03% African American, 0.43% Native American, 0.16% Asian, 0.40% from other races, and 0.46% from two or more races. Hispanic or Latino of any race were 1.18% of the population.

Of the 1,554 households 30.4% had children under the age of 18 living with them, 54.0% were married couples living together, 7.7% had a female householder with no husband present, and 36.4% were non-families. 33.8% of households were one person and 17.8% were one person aged 65 or older. The average household size was 2.33 and the average family size was 3.01.

The age distribution was 26.7% under the age of 18, 5.7% from 18 to 24, 24.8% from 25 to 44, 20.4% from 45 to 64, and 22.3% 65 or older. The median age was 40 years. For every 100 females, there were 84.1 males. For every 100 females age 18 and over, there were 80.9 males.

The median household income was $30,815 and the median family income  was $40,063. Males had a median income of $28,614 versus $18,627 for females. The per capita income for the city was $15,998. About 5.0% of families and 8.5% of the population were below the [[poverty line]], including 3.4% of those under age 18 and 12.3% of those age 65 or over.