According to the United States Census Bureau, the city has a total area of 16.5 square miles (42.7 km²), of which, 15.8 square miles (40.9 km²) of it is land and 0.7 square miles (1.7 km²) of it (4.07%) is water.
Northeastern Wapello County contains large deposits of coal, and there are also large deposits of clay in the region. These were very important in the industrial development of Ottumwa.
As of the census of 2000, there were 24,998 people, 10,383 households, and 6,530 families residing in the city. The population density was 1,582.2 people per square mile (610.9/km²). There were 11,038 housing units at an average density of 698.6/sq mi (269.7/km²). The racial makeup of the city was 95.33% White, 1.27% African American, 0.33% Native American, 0.78% Asian, 0.02% Pacific Islander, 1.38% from other races, and 0.89% from two or more races. Hispanic or Latino of any race were 2.76% of the population.
There were 10,383 households out of which 28.1% had children under the age of 18 living with them, 47.8% were married couples living together, 11.3% had a female householder with no husband present, and 37.1% were non-families. 31.1% of all households were made up of individuals and 14.8% had someone living alone who was 65 years of age or older. The average household size was 2.31 and the average family size was 2.88.
In the city the population was spread out with 22.9% under the age of 18, 10.9% from 18 to 24, 26.1% from 25 to 44, 21.2% from 45 to 64, and 19.0% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 92.8 males. For every 100 females age 18 and over, there were 88.6 males.
The median income for a household in the city was $30,174, and the median income for a family was $37,302. Males had a median income of $31,222 versus $20,934 for females. The per capita income for the city was $16,040. About 10.9% of families and 15.2% of the population were below the poverty line, including 21.3% of those under age 18 and 8.2% of those age 65 or over.
Amtrak, the national passenger rail system, provides service to Ottumwa, operating its California Zephyr daily in both directions between Chicago, Illinois, and Emeryville, California, across the San Francisco Bay from San Francisco.
The Norfolk Southern Railway has trackage rights over the BNSF through Ottumwa.
(See the Amtrak entry under Transportation (below).)
Ottumwa is known for having many different historical locations around its downtown area. In central downtown the Canteen in the Alley has been a stopping point for Ottumwans since the 1920s. Located south of Richard Nixon's residence during his stay in the 1940s many other famous patrons since Nixon have also been seen eating a "Canteen" (a loose meat sandwich similar to a Maid-Rite). On the sitcom Roseanne starring Roseanne Barr and briefly Tom Arnold, Roseanne's restaurant was based on the Canteen Lunch in the Alley.