According to the United States Census Bureau, the city has a total area of 2.4 square miles (6.1 km²), all of it land.
O'Neill is the official "Irish Capitol of Nebraska". Cattle, potatoes, tomatoes, soybeans and corn are the major products produced near this town.
There were 1,554 households out of which 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 all households were made up of individuals and 17.8% had someone living alone who was 65 years of age or older. The average household size was 2.33 and the average family size was 3.01.
In the city the population was spread out with 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% who were 65 years of age 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 income for a household in the city was $30,815, and the median income for a family 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.
Father Edward Flanagan, who went on to establish Boys Town, first served as a Catholic priest at St. Patrick's Catholic Church in O'Neill.
For now, O'Neill houses the world's largest permanent shamrock. Made of colored concrete, it was installed in the main intersection of 4th & Douglas in 2000. Saint Patrick's Day and the Summerfest in July are the town's main celebrations.
The local radio station is KBRX, 102.9FM, 1350AM. Both bands play country music most of the time, though the AM band also has blocks of polka ("milking music") in the morning and oldies in the evening. The Holt County Independent is O'Neill's local newspaper.