Oak Forest is considered part of the Chicago metropolitan area. It is mostly surrounded by Cook County Forest Preserves (hence the name). Some neighboring communities of Oak Forest include Crestwood to the north, Midlothian to the northeast, Markham to the east, Country Club Hills to the southeast, Tinley Park to the southwest, Orland Park to the west, beyond Bachelor's Grove Cemetery and Forest Preserve is Palos Heights to the northwest.
According to the United States Census Bureau, the city has a total area of 5.7 square miles (14.8 km²), of which, 5.7 square miles (14.6 km²) of it is land and 0.1 square miles (0.1 km²) of it (0.88%) is water.
As of the 2000 census, there were 28,051 people, 9,785 households, and 7,338 families residing in the city. The population density was 4,965.4 people per square mile (1,916.9/km²). There were 10,022 housing units at an average density of 1,774.0/sq mi (684.9/km²). The racial makeup of the city was 90% White, 4% African American.
The top five ancestries reported in Oak Forest as of the 2000 census were Irish (26.3%), German (25.1%), Polish (18.3%), Italian (13.0%) and Dutch (5.1%).
There were 9,785 households out of which 36.9% had children under the age of 18 living with them, 61.0% were married couples living together, 9.7% had a female householder with no husband present, and 25.0% were non-families. 20.7% of all households were made up of individuals and 5.9% had someone living alone who was 65 years of age or older. The average household size was 2.81 and the average family size was 3.30.
In the city the population was spread out with 26.0% under the age of 18, 9.1% from 18 to 24, 31.0% from 25 to 44, 24.7% from 45 to 64, and 9.2% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 99.3 males. For every 100 females age 18 and over, there were 97.7 males.
The median income for a household in the city was $60,073, and the median income for a family was $68,862. Males had a median income of $48,808 versus $30,137 for females. The per capita income for the city was $23,487. About 2.7% of families and 3.6% of the population were below the poverty line, including 3.1% of those under age 18 and 3.4% of those age 65 or over.
