- O'Fallon (Population: 21,910)
- Oak Brook (Population: 8,702)
- Oak Forest (Population: 28,051)
- Oak Grove (Population: 1,318)
- Oak Lawn (Population: 55,245)
- Oak Park (Population: 52,524)
- Oakbrook Terrace (Population: 2,300)
- Oakwood Hills (Population: 2,194)
- Oakwood (Population: 1,502)
- Oblong (Population: 1,580)
- Odell (Population: 1,014)
- Odin (Population: 1,122)
- Oglesby (Population: 3,647)
- Okawville (Population: 1,355)
- Olney (Population: 8,631)
- Olympia Fields (Population: 4,732)
- Onarga (Population: 1,438)
- Oquawka (Population: 1,539)
- Oregon (Population: 4,060)
- Orion (Population: 1,713)
- Orland Hills (Population: 6,779)
- Orland Park (Population: 51,077)
- Oswego (Population: 13,326)
- Ottawa (Population: 18,307)
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