O'fallon leans Republican by roughly 16 points: about 42% of voters vote Democratic and 58% Republican.
About 79% of adults in O'fallon typically vote, above the U.S. average of about 62%. Among adults in O'fallon, ~33% vote Democratic, ~46% Republican, and ~21% don't vote. The map below shows estimated turnout by block group.
How O'fallon compares
Among cities within 25 miles, O'fallon leans more Republican than 77 of 145 neighbors.
Politically, O'fallon sits close to the rest of Missouri.
Politics vary noticeably by neighborhood within O'fallon. The northwest side is the most Republican-leaning (R+29) and the southeast side is the least Republican-leaning (R+11), a spread of about 18 points.
Why O'fallon leans the way it does
This analysis examined 14,881 data points per city to find what predicts political lean and turnout. The items below are a few correlations that stood out for O'fallon, not a ranked or complete list of what matters most.
O'fallon votes Republican even though it is densely developed (about 86%, far above the Missouri average of 22%). State and regional patterns outweigh the Democratic lean that density usually predicts here.
Preventive-care access and voter turnout
Places with strong routine preventive-care access tend to turn out at a higher rate; O'fallon, MO sits above the national average on this measure. Dental visits do not drive turnout; the rate reflects income, insurance, and healthcare access, which line up with who votes.
Why turnout in O'fallon looks the way it does
Turnout in O'fallon sits close to the national pattern. Routine healthcare access, homeownership, education, and food security all land near their national averages here. Learn more about the findings and methodology on the political spectrum map.
Nearby Cities
- Dardenne Prairie, MO R+17
- Cottleville, MO R+16
- Lake St. Louis, MO R+16
- St. Paul, MO R+32
- St. Peters, MO R+9
- Josephville, MO R+35
- Weldon Spring Heights, MO R+31
- Weldon Spring, MO R+23
- Wentzville, MO R+24
Cities with Similar Populations
- Rochester Hills, MI D+5
- San Pedro, CA D+27
- New Rochelle, NY D+35
- Mililani, HI D+14
- Stockbridge, GA D+40
- Flower Mound, TX R+16
- Mansfield, OH R+14
- Simpsonville, SC R+22
- Winchester, VA R+10
- Caldwell, ID R+37
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Sources and methodology
Precinct-level voting records used to fit the model come from Missouri Secretary of State, Elections, distributed by the Voting and Election Science Team. Demographic inputs come from the U.S. Census Bureau (ACS 5-year estimates and the 2020 Decennial Census). Health and environmental inputs come from the CDC (PLACES and the Environmental Justice Index). Land cover comes from the USGS and EPA. Election-day and lead-up weather come from PRISM 4km daily grids and the NOAA Global Historical Climatology Network. Mail-voting and election-administration patterns come from the MIT Election Lab's Survey of the Performance of American Elections. Block-group crime detail comes from CrimeGrade. Internet data and modeling support provided by ISPreports.org.
Modeling and analysis by the BestNeighborhood data science team. Full methodology and findings: political spectrum map.
Methodology reviewed by the BestNeighborhood data team. Last updated May 2026.