Oakview (Marshall County, Mississippi)
Oakwold Plantation House (Avoyelles Parish, Louisiana)
Oakwood (Anne Arundel County, Maryland)
Oakwood (Burlington County, New Jersey)
Oakwood (Salt Lake County, Utah)
Oakwood (Licking County, Ohio)
Oakwood (Howard County, Missouri)
Oakwood (Adams County, Mississippi)
Oakwood (Richland County, South Carolina)
Oakwood (Fauquier County, Virginia)
Oaky Grove (Wake County, North Carolina)
Oaterhoudt Stone House (Ulster County, New York)
Oates House (Henry County, Alabama)
Oatlands (Loudoun County, Virginia)
Obadiah Dingee House (New Castle County, Delaware)
Obediah Barber Homestead (Ware County, Georgia)
Obediah Farrar House (Lee County, North Carolina)
Obediah Shirley House (Anderson County, South Carolina)
Historic home in Honea Path, SC
Obee House (Lake County, Illinois)
Oberg--Metcalf House (Sanpete County, Utah)
Obligation (Anne Arundel County, Maryland)
Ocean County Jail (Ocean County, New Jersey)
Ockers, Jacob, House (Suffolk County, New York)
Octagon Cottage (Barren County, Kentucky)
Octagon House (Muscogee County, Georgia)
Octagon House (Jefferson County, Wisconsin)
Octagon House (Fond du Lac County, Wisconsin)
Octagon House that was once a stop on the Underground Railroad.
Octagon House (Laurens County, South Carolina)
Octagon House (Fairfield County, Connecticut)
Octagon House (Cook County, Illinois)
Octagon House (Fairfield County, Connecticut)
destroyed by fire in 1985
Octagon House (Hampden County, Massachusetts)
Octagon House (Middlesex County, Massachusetts)
Octagon, The (Washington, District of Columbia)
Octagon, The (Seneca County, Ohio)
Odell House (Westchester County, New York)
Odessa Ice Cream Company Building (Lafayette County, Missouri)
Oehler Brick Buildings (St. Louis, Missouri)
Officers Row, Fort Vancouver Barracks (Clark County, Washington)
Ogden House (Delaware County, Pennsylvania)
Ogden Mallory House (Erie County, Ohio)
Ogden, David, House (Fairfield County, Connecticut)
Ogilvie Moses Farmhouse (Livingston County, New York)
Oglebay Mansion Museum (Ohio County, West Virginia)
Oglesby-Conrad (Breckinridge County, Kentucky)
Ohio Moline Plow Company Building (Franklin County, Ohio)
Ohlandt Newlyweds House (San Francisco County, California)
Ohlman-Shannon House (Yankton County, South Dakota)
Okolona (New Castle County, Delaware)
Olaf Lee House (Ramsey County, Minnesota)
Olana (Columbia County, New York)
Olcovich--Meyers House (Carson City, Nevada)
Old Adobe Patio (Pima County, Arizona)
Old Alexander House (Columbia County, Arkansas)
Old Backs House (Orange County, California)
Old Bailey House (Maui County, Hawaii)
Old Berrien County Courthoue Complex (Berrien County, Michigan)
Old Bloomfield (Talbot County, Maryland)
Old Borges Ranch (Contra Costa County, California)
Old Brass (Beaufort County, South Carolina)
Old Brick House (Pasquotank County, North Carolina)
Old Brulay Plantation (Cameron County, Texas)
Old Cann Mansion House (New Castle County, Delaware)
Old Chelmsford Garrison House Complex (Middlesex County, Massachusetts)
Old Chiswell Place (Montgomery County, Maryland)
Old Corner Bookstore (Suffolk County, Massachusetts)
Old Dawson Place (Muscogee County, Georgia)
Old District Parsonage (Hampshire County, West Virginia)
Old Dutch Church Parsonage (Ulster County, New York)
Old Elmspring Hutterite Colony (Hutchinson County, South Dakota)
Old Executive Mansion (Dane County, Wisconsin)
Old Fentress County Jail (Fentress County, Tennessee)
Old Fernald--Laughton Memorial Hospital (Seminole County, Florida)
Old Ford Dairy (New Castle County, Delaware)
Old Forge Farm (Frederick County, Virginia)
Old Fort House (Washington County, New York)
Old Fort House (Lowndes County, Mississippi)
Old French House (Knox County, Indiana)
Fine example of French colonial construction techniques
Old Funeral Palor (Erie County, Ohio)
Old Gaissert Homeplace (Spalding County, Georgia)
Old Garrison House (Essex County, Massachusetts)
Old German Consulate (Jefferson County, Washington)
Old Governor's Mansion (Baldwin County, Georgia)
Old Governor's Mansion (Yavapai County, Arizona)
Old Governor's Mansion (Franklin County, Kentucky)
Old Governor's Mansion (Franklin County, Ohio)
Old Headquarters (Coconino County, Arizona)
Old Hickory (Pointe Coupee Parish, Louisiana)
Old Homestead (Lawrence County, Pennsylvania)
Old Homestead (Monroe County, Mississippi)
Old House Plantation (Charleston County, South Carolina)
Old House, The (Suffolk County, New York)
Old Indian Agency House (Columbia County, Wisconsin)
Old Indiana County Jail and Sheriff's Office (Indiana County, Pennsylvania)
Old Jail (Hancock County, Tennessee)
Old Jail (Wilkes County, Georgia)
Old Jib's Corner (Erie County, Ohio)
Old Kentuck (Monmouth County, New Jersey)
Old Kentucky Tavern (Trimble County, Kentucky)
Old Log Cabin (Sarpy County, Nebraska)
Old Louisiana Governor's Mansion (East Baton Rouge Parish, Louisiana)
Old Manse (Middlesex County, Massachusetts)
Old Mansion (Caroline County, Virginia)
Old Merchant's House (New York County, New York)
Old Mill House (Scott County, Iowa)
Old Miller Hospital (Shiawassee County, Michigan)
Old Pickens Jail (Pickens County, South Carolina)
Early 20th Century county jail
Old Pierce House (Harrison County, Texas)
Old President's Mansion (Lee County, Alabama)
Was constructed in 1915 as the residence for the president of Auburn University (then the Alabama Polytechnic Institute).
Old Rectory (Bedford County, Virginia)
Old Rock House (Lincoln County, Missouri)
Old Rockport Hutterite Colony (Hanson County, South Dakota)
Old Saybrook South Green (Middlesex County, Connecticut)
Old Spink Colony (Spink County, South Dakota)
Old St. Wenceslaus Catholic Parish House (Bon Homme County, South Dakota)
Old Stone House (Monongalia County, West Virginia)
Old Stone House (Washington, District of Columbia)
Old Stone House (Richmond, Virginia)
Old Stone House (Bath County, Virginia)
Old Stone House (Fairfield County, South Carolina)
Historic home near Winnsboro, SC
Old Stone Tavern (Kent County, Delaware)
Old Strother Place (Saluda County, South Carolina)
Historic home near Saluda, SC
Old Swede's House (Berks County, Pennsylvania)
Old Timbers (Ripley County, Indiana)
Old Town (Williamson County, Tennessee)
Old Town Plantation (Edgecombe County, North Carolina)
Old Trees (Madison County, New York)
Old Turner Place (Henry County, Virginia)
Old Warrick County Jail (Warrick County, Indiana)
Old Wash Place (Anderson County, Kentucky)
Old Watson Homestead House (Monongalia County, West Virginia)
Old Westbury Gardens (Nassau County, New York)
Old Winters Ranch/Winters Mansion (Washoe County, Nevada)
Oldfields (Marion County, Indiana)
Oldfields (Marion County, Indiana)
Oldham, Mary and Frank House (Ellis County, Texas)
Ole and Carris Johnson Ranch (Meade County, South Dakota)
Ole and Elizabeth Finstad Homesite (Bosque County, Texas)
Ole and Polly Oleson Farmhouse (Washington County, Oregon)
Ole Arilsen House (Sanpete County, Utah)
Ole Carlson House (Dodge County, Minnesota)
Ole K. Roe House (Dane County, Wisconsin)
Ole Mickelson Cabin (Grays Harbor County, Washington)
Oliphant House (Sabine County, Texas)
Oliphant--Walker House (Travis County, Texas)
Olivas Adobe (Ventura County, California)
Olive Hill (Chesterfield County, Virginia)
Olive Place (Codington County, South Dakota)
Oliver and Gertrude Lundquist House (Burleigh County, North Dakota)
Oliver and Margaret Jeffrey House (Multnomah County, Oregon)
Oliver Barrett House (Dutchess County, New York)
Oliver Beers House (Marion County, Oregon)
Oliver Blood House (Ravalli County, Montana)
Oliver Brewster House (Orange County, New York)
Oliver H. Hovda House (Stillwater County, Montana)
Oliver H. Kelley Homestead (Sherburne County, Minnesota)
Oliver House (Clay County, Arkansas)
Oliver House (Kemper County, Mississippi)
Oliver John Harmon House (Carbon County, Utah)
Oliver N. Ainsworth House (Lawrence County, South Dakota)
Oliver O. Stokes House (Harding County, South Dakota)
Oliver P. Fannin House (Floyd County, Georgia)
Oliver P. Jones House (Bastrop County, Texas)
Oliver P. Morton House (Wayne County, Indiana)
Oliver Rice House (Fulton County, New York)
Oliver Sturges House (Chatham County, Georgia)
Oliver White Tavern (Tolland County, Connecticut)
Oliver Wiswall House (Columbia County, New York)
Oliver-Leming House (Cape Girardeau County, Missouri)
Ollinger-Cobb House (Santa Rosa County, Florida)
Olmstead Manor (McKean County, Pennsylvania)
Historic mansion converted to a retreat center
Olmstead Place State Park (Kittitas County, Washington)
Olney (Harford County, Maryland)
Olney, Capt. Stephen, House (Providence County, Rhode Island)
Olof Johnson House (Henry County, Illinois)
Olson House (Knox County, Maine)
Oluf Larsen House (Sanpete County, Utah)
Olvida Penas (Monterey County, California)
Olzie Whitehead Williams House (Wilson County, North Carolina)
Omer/Pound House (Jefferson County, Kentucky)
On The Hill (Mecklenburg County, Virginia)
Onderdonk House (Rockland County, New York)
Onderdonk, Horatio Gates, House (Nassau County, New York)
One-hundred-and-one Ranch (Kay County, Oklahoma)
Oneida Community Mansion House (Madison County, New York)
Onon B. and Betsy Dahle House (Dane County, Wisconsin)
Ooten House (Pawnee County, Kansas)
Opp Cottage (Montgomery County, Alabama)
Opposition House (Middlesex County, Massachusetts)
Ora Holland House (Dubuque County, Iowa)
Private Residence closed to the public
Ora Pelton House (Kane County, Illinois)
Orange Grove (Sumter County, South Carolina)
Orange Grove Court (Los Angeles County, California)
Orange Grove Plantation House (Terrebonne Parish, Louisiana)
Orange Lawn (Sonoma County, California)
Orator F. Woodward Cottage (Wyoming County, New York)