It is named for Benjamin O'Fallon, a nephew of Captain William Clark and an Indian agent for the upper Missouri region from 1823 to 1827.

[citation needed] As of the 2010 census, there were 2,890 people, 1,233 households, and 810 families living in the county.

The racial makeup of the county was 97.4% white, 0.6% Asian, 0.4% American Indian, 0.1% Pacific islander, 0.1% black or African American, 0.2% from other races, and 1.3% from two or more races.

In terms of ancestry, 53.0% were German, 24.8% were Irish, 12.1% were Norwegian, 10.6% were English, 5.3% were Scotch-Irish, and 0.4% were American.

Males had a median income of $41,570 versus $30,000 for females.