Ochiltree County was organized in 1876.
As of the census of 2000, there were 9,006 people, 3,261 households, and 2,488 families residing in the county. The population density was 10 people per square mile (4/km²). There were 3,769 housing units at an average density of 4 per square mile (2/km²). The racial makeup of the county was 86.2 percent White, 0.13 percent Black or African American, 0.94 percent Native American, 0.39 percent Asian, 0.01 percent Pacific Islander, 10.28 percent from other races, and 2.04 percent from two or more races. 13.79 percent of the population were Hispanic or Latino of any race.
There were 3,261 households, of which 40.9 had children under the age of 18, 64 percent were married couples living together, 7.9 percent had a female householder with no husband present, and 23.7 percent were non-families. Of unmarried partner households, 89.5 percent were homosexual, 6.3 percent were same-sex male, and 4.2 percent were same-sex female.
Twenty-one percent of all households were made up of individuals and 9.30 percent had someone living alone who was 65 years of age or older. The average household size was 2.74 and the average family size was 3.18.
In the county, the population was spread out with 30.6 percent under the age of 18, 8.4 percent from 18 to 24, 28.7 percent from 25 to 44, 20.7 percent from 45 to 64, and 11.7 percent who were 65 years of age or older. The median age was 34 years. For every 100 females there were 99.8 males. For every 100 females age 18 and over, there were 96.9 males.