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- The O'Brien Corporation — the varnish works on Washington Street Businesses
- The O'Brien Corporation After 1983 Businesses
- Oaklawn: the Mennonite center that holds the designation Mental Health Institutions
- October 1973: the great wave, and where Indiana's center lay UFO Sightings
- The October tornadoes · 1967's single, and the outbreak of 24 October 2001 Wars, Unrest & Disasters
- The Odd Fellows Private Clubs
- The Odd Fellows: Lodge No. 29 and the Rebekah degree's author Lodges & Fraternal Orders
- The official books: Erskine (1918), Gordon (1922), and the Commission report (2026) Sources
- Okapi Arrive; the Chimpanzees Leave Zoo
- The Old Courthouse museum — eighty-eight years in a courtroom building Museums
- The Old Settlers' Stock: The Stories Howard Collected Folklore
- "The oldest continuously operating school in the city" — Saint Joseph Grade School Famous Stories & the Record
- "The oldest zoo in Indiana" — and the zoo's own correction of it Famous Stories & the Record
- Oliver addenda — the 1880 count, and the power plant's dates Businesses
- The Oliver Chilled Plow Tractor: the plow works builds twenty tractors Tractors
- Oliver Chilled Plow Works — the plow that broke the prairie Businesses
- The Oliver Fortune: Millions in Print, No Total in Probate Great Fortunes
- The Oliver Hotel Hotels
- The Oliver House Hotels
- The Oliver Opera House Theaters
- The Oliver Opera House rooms Libraries
- The Oliver Theatre Theaters
- The Oliver works in 1880: the plant as a place Industrial Revolution
- The Olivers: the plow, the presidency, and Copshaholm Notable People
- Olivet A.M.E. · the mother church Black History
- Ollie Bejma: The Studebaker Team's Own Major Leaguer Sports Players
- The Olympians of This Ground Sports Players
- On the water: tested, and mostly a negative Racing
- The one Bitcoin prosecution Lottery & Crypto
- One car, built in secret: the Bendix SWC of 1934 Automobiles
- One River, Two Cities: The Riverwalk Whole Trails
- The ones that closed · and one that was never this county's Cold Cases
- The only murder on the campus · Helen Tobolski Cold Cases
- The open-housing fight · William Morris, the realtors' board, and the ordinance of March 1968 Black History
- The opposition: what the churches and the critics said Lodges & Fraternal Orders
- Orchards and small fruit Farms & Crops
- An orchestra built twice in one year: the Symphony, 1932 Culture
- The Order of Owls: a national secret order founded in South Bend Lodges & Fraternal Orders
- The Order of the Owls: founded in South Bend Private Clubs
- The original plat: a polygon of quarter-acre lots Neighborhoods
- The Orpheum Theatre Theaters
- The Osco murders · Christopher Allen's four courtrooms, 1990–2006 Notable Murder Cases
- The Osco murders — Christopher Allen's post-dismissal record, and the case's standing in November 2026 Notable Murder Cases
- The osprey: the county's densest nesting ground Wildlife
- The other chilled plows — the Bissell and Economist works Businesses
- The other craft rooms: Crooked Ewe, Studebaker Brewing, and the Ivy Alley brewery Wineries & Distilleries
- The other dams on the reach: a printed negative and the boundary line The St. Joseph River
- The Other Districts — the Register's Survey of the Neighborhoods Architecture
- The other interurban: the Northern Indiana Railway Railroads
- The other side of the ledger: the bust, at its true size Dot-Com Era
- The other state roads: the Vistula Road and the Chicago Road Main Roads & Corridors
- The otter: out by 1942, back by spread Wildlife
- Our Lady of Hungary Parish Churches & Religion
- Our Next Chapter Libraries
- Over $12,000 · Darius Jiggetts and the television identification Bank Robberies
- The Overhand Era and the Tie That Was Not a Pennant: 1948–1949 South Bend Blue Sox
- Ox-Head Prairie, named for a head of monstrous size Strange but True