Ocala Historic District (Marion County, Florida)
Ocala Union Station (Marion County, Florida)
Occidental Hotel (Sedgwick County, Kansas)
Occidental Life Insurance Company Building (Wake County, North Carolina)
Occoquan Historic District (Prince William County, Virginia)
Occum Hydroelectric Plant and Dam (New London County, Connecticut)
Ocean City City Hall (Cape May County, New Jersey)
Ocean City Residential Historic District (Cape May County, New Jersey)
Ocean County Courthouse (Ocean County, New Jersey)
Ocean County Jail (Ocean County, New Jersey)
Ocean Drive Historic District (Newport County, Rhode Island)
Ocean Forest Country Club (Horry County, South Carolina)
Ocean Hall (St. Mary's County, Maryland)
Ochre Point-Cliffs Historic District (Newport County, Rhode Island)
Ochs Building (Hamilton County, Tennessee)
Ochsner Hospital (La Plata County, Colorado)
Ocilla Public School (Irwin County, Georgia)
Ockers, Jacob, House (Suffolk County, New York)
Ocmulgee National Monument (Bibb County, Georgia)
Ocoee Christian Church (Orange County, Florida)
Ocoee No. 1 Hydroelectric Station (Polk County, Tennessee)
Ocoee Street Historic District (Bradley County, Tennessee)
Oconaluftee Baptist Church (Swain County, North Carolina)
Oconee Station and Richards House (Oconee County, South Carolina)
Historic frontier outpost
Oconomowoc City Hall (Waukesha County, Wisconsin)
Oconto County Courthouse (Oconto County, Wisconsin)
Oconto Main Post Office (Oconto County, Wisconsin)
Ocracoke Historic District (Hyde County, North Carolina)
Ocracoke Light Station (Hyde County, North Carolina)
Octagon Building (Santa Cruz County, California)
Octagon Hall (Simpson County, Kentucky)
Octagon House (Jefferson County, Wisconsin)
Octagon House (Fairfield County, Connecticut)
destroyed by fire in 1985
Octagon House (Hampden County, Massachusetts)
Octagon, The (New York County, New York)
Octagon, The (Washington, District of Columbia)
Octagon, The (Seneca County, Ohio)
Octagonal Schoolhouse (Kent County, Delaware)
Octorara Farm (Cecil County, Maryland)
Odd Fellows Block (Grand Forks County, North Dakota)
Odd Fellows Block (Androscoggin County, Maine)
Odd Fellows Block (Union County, Iowa)
Odd Fellows Building (Deuel County, South Dakota)
Odd Fellows Building (Middlesex County, Massachusetts)
Odd Fellows Building (Wake County, North Carolina)
Odd Fellows Building (Multnomah County, Oregon)
Odd Fellows Building (Pike County, Kentucky)
Odd Fellows Building (Daviess County, Kentucky)
Odd Fellows Building and Auditorium (Fulton County, Georgia)
Odd Fellows Hall (Alexandria, Virginia)
Odd Fellows Hall (Humboldt County, California)
Odd Fellows Hall (Kenton County, Kentucky)
Odd Fellows Hall (Baltimore, Maryland)
Odd Fellows Hall (Sheridan County, Wyoming)
Odd Fellows Hall (Middlesex County, Massachusetts)
Odd Fellows Hall (Orange County, California)
Odd Fellows Hall (New York County, New York)
Odd Fellows Hall (Beaver County, Utah)
Odd Fellows Hall (Jones County, Iowa)
Odd Fellows Hall (Scioto County, Ohio)
Odd Fellows Hall (Franklin County, Ohio)
Odd Fellows Home District (Clay County, Missouri)
Odd Fellows Lodge (Harford County, Maryland)
Odd Fellows Lodge (Wayne County, North Carolina)
Odd Fellows Opera Block (Fulton County, Illinois)
Odd Fellows Temple (Columbiana County, Ohio)
Odd Fellows' Hall (Essex County, Massachusetts)
Odd Fellows' Home (Worcester County, Massachusetts)
demolished in 2014
Oddfellow's Hall (Providence County, Rhode Island)
Oddfellows Home (Graham County, Arizona)
Odell Building (Whiteside County, Illinois)
Odell-Locke-Randolph Cotton Mill (Cabarrus County, North Carolina)
Odeon Theater (Quay County, New Mexico)
Odessa Ice Cream Company Building (Lafayette County, Missouri)
Odessa Jail (Big Stone County, Minnesota)
Oehler Brick Buildings (St. Louis, Missouri)
Oeldorf Building/Wetherell's Jewelers (Wood County, West Virginia)
Oella Historic District (Baltimore County, Maryland)
Offenhauser Insurance Building (Bowie County, Texas)
Office Hall (King George County, Virginia)
Officer Farmstead (Overton County, Tennessee)
Officer's Quarters (Pima County, Arizona)
Officers Row, Fort Vancouver Barracks (Clark County, Washington)
Officers' Row Historic District (Kitsap County, Washington)
Ogden Apartment Hotel (Hennepin County, Minnesota)
Ogden High School (Weber County, Utah)
Ogden Theatre (Denver County, Colorado)
Ogden Union Station (Weber County, Utah)
Ogden, David, House (Fairfield County, Connecticut)
Ogden/Weber Municipal Building (Weber County, Utah)
Ogilvie Building (Huron County, Michigan)
Ogilvie Hardware Company Building (Caddo Parish, Louisiana)
Ogle County Courthouse (Ogle County, Illinois)
Romanesque Revival, red brick structure designed by George O. Garnsey
Oglebay Hall (Monongalia County, West Virginia)
Oglebay Mansion Museum (Ohio County, West Virginia)
Oglesby-Conrad (Breckinridge County, Kentucky)
Oglethorpe Avenue Historic District (Clarke County, Georgia)
Ogontz Hall (Philadelphia County, Pennsylvania)
Ohave Shalom Synagogue (Sullivan County, New York)
Ohev Sholem Synagogue (New London County, Connecticut)
Ohev Sholom Temple (Cabell County, West Virginia)
Ohio City Preservation District (Cuyahoga County, Ohio)
Ohio Farm Bureau Federation Offices (Franklin County, Ohio)
Ohio Lantern Company (Seneca County, Ohio)
Ohio Moline Plow Company Building (Franklin County, Ohio)
Ohio National Guard Armory (Hamilton County, Ohio)
Ohio Oil Company Building (Natrona County, Wyoming)
Ohio One--Ohio Edison (Mahoning County, Ohio)
Ohio Power Company (Muskingum County, Ohio)
Ohio State Arsenal (Franklin County, Ohio)
Ohio State Normal College At Kent (Portage County, Ohio)
Ohio State Reformatory (Richland County, Ohio)
Ohio Theatre (Lucas County, Ohio)
Ohio Theatre (Allen County, Ohio)
Ohio Theatre (Richland County, Ohio)
Ohio Valley Clay Company (Jefferson County, Ohio)
Ohiowa Public School (Fillmore County, Nebraska)
Oil City Armory (Venango County, Pennsylvania)
Oil City Downtown Commercial Historic District (Venango County, Pennsylvania)
Oil City North Side Historic District (Venango County, Pennsylvania)
OKLA Theater (Pittsburg County, Oklahoma)
Oklahoma City University (Oklahoma County, Oklahoma)
Oklahoma County Home for Girls (Oklahoma County, Oklahoma)
Oklahoma Hardware Building (Oklahoma County, Oklahoma)
Oklahoma Natural Gas Company Building (Tulsa County, Oklahoma)
Oklahoma Odd Fellows Home at Checotah (McIntosh County, Oklahoma)
Oklahoma Presbyterian College (Bryan County, Oklahoma)
Okmulgee Colored Hospital (Okmulgee County, Oklahoma)
Okmulgee County Courthouse (Okmulgee County, Oklahoma)
Okmulgee Downtown Historic District (Okmulgee County, Oklahoma)
Okmulgee Public Library (Okmulgee County, Oklahoma)
Okolona College (Chickasaw County, Mississippi)
Okolona Historic District (Chickasaw County, Mississippi)
Oktaha School (Muskogee County, Oklahoma)
Olaf Stordahl Barn (Kingsbury County, South Dakota)
Olana (Columbia County, New York)
Olcott, John E., House (Middlesex County, Massachusetts)
Olcovich--Meyers House (Carson City, Nevada)
Old A.C.L. Union Depot (Putnam County, Florida)
Old Akron Post Office (Summit County, Ohio)
Old Alexander House (Columbia County, Arkansas)
Old Allegany County Courthouse (Allegany County, New York)
Old Allegheny Rows Historic District (Allegheny County, Pennsylvania)
Old Amherst Freight Depot (Lorain County, Ohio)
Old Arabi Historic District (St. Bernard Parish, Louisiana)
Old Armory (Williams County, North Dakota)
Old Armory --Vermillion (Clay County, South Dakota)
Old Athens, Alabama Main Post Office (Limestone County, Alabama)
Was built in 1931.
Old Atoka State Bank (Atoka County, Oklahoma)
Old Auburn Historic District (Placer County, California)
Old Baker County Courthouse (Baker County, Florida)
Old Bakery (Travis County, Texas)
Old Bank of Fayetteville Building (Washington County, Arkansas)
Old Barnhill Building (Moniteau County, Missouri)
Old Bartow County Courthouse (Bartow County, Georgia)
Old Batesburg Grade School (Lexington County, South Carolina)
Old Batesburg-Leesville High School (Lexington County, South Carolina)
Old Beechwold Historic District (Franklin County, Ohio)
Old Bell Telephone Building (Mississippi County, Arkansas)
Old Belleair Town Hall (Pinellas County, Florida)
Old Berrien County Courthoue Complex (Berrien County, Michigan)
Old Bethany Church (Brooke County, West Virginia)
Old Bethel United Methodist Church (Charleston County, South Carolina)
Old Biology Hall (Madison County, New York)
Old Bittermann Building (Vanderburgh County, Indiana)
Old Bloomfield (Talbot County, Maryland)
Old Bluff Presbyterian Church (Cumberland County, North Carolina)
Old Bnai Zion Synagogue (El Paso County, Texas)
Old Bradford County Courthouse (Bradford County, Florida)
Old Brazoria County Courthouse (Brazoria County, Texas)
Old Brick Church (Windham County, Vermont)
Old Brick Church (Surry County, Virginia)
Old Brick House (Pasquotank County, North Carolina)
Old Brick Mill (Floyd County, Georgia)
Old Brick Post Office (Maricopa County, Arizona)
Old Brick Presbyterian Church (Colbert County, Alabama)
Old Brick Store (New Castle County, Delaware)
Old Brick Warehouse (Marion County, South Carolina)
Old Brickyard Place (Claiborne County, Mississippi)
Old Bridge Historic District (Middlesex County, New Jersey)
Old Bridgeville Fire House (Sussex County, Delaware)