O'Neill is a city in Holt County, Nebraska, United States. The population was 3,551 at the 2010 census. It is the county seat of Holt County. O'…O'Neill is a city in Holt County, Nebraska, United States. The population was 3,551 at the 2010 census. It is the county seat of Holt County. O'Neill was platted in 1874. It was named for one of its founders, John O'Neill. O'Neill was originally settled largely by Irish immigrants.