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 Homestead (Contra Costa County, California)
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 Manse (Orange County, Virginia)
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 Bohman House (Latah County, Idaho)
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 McLeod House (Missoula County, Montana)
Olive Place (Codington County, South Dakota)
Oliver and Gertrude Lundquist House (Burleigh County, North Dakota)
Oliver and Margaret Jeffrey House (Multnomah County, Oregon)
Oliver Beers House (Marion County, Oregon)
Oliver Blood House (Ravalli County, Montana)
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. Howard House (Clark County, Washington)
Two story wood frame house built in 1879
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)
Olsen House (Lewis and Clark County, Montana)
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)
Orchards, The (Bennington County, Vermont)
Orem's Delight (Talbot County, Maryland)
Oren Law House and Outbuildings (Bear Lake County, Idaho)
Orgone Energy Observatory (Franklin County, Maine)
Orin Clark House (De Kalb County, Indiana)
Orin Jordan House (Los Angeles County, California)
Orin Mallette Cabin (Taos County, New Mexico)
Orin Porter House (Summit County, Ohio)
Orin Savage Cottage (Franklin County, New York)
Orion Clemens House (Carson City, Nevada)
Orlan A. Hayward House (Minnehaha County, South Dakota)
Orland P. Bassett House (Du Page County, Illinois)
Orland Selby House (Lake County, Ohio)
Orlando (Essex County, Massachusetts)
Orlando Brice House (Eau Claire County, Wisconsin)
Stately Georgian Revival residence
Orlando Coolidge House (Jackson County, Oregon)
Orlando Humason House (Wasco County, Oregon)
Orlando W. Warner House (Grand County, Utah)
Orman-Adams House (Pueblo County, Colorado)
Ormond Plantation House (St. Charles Parish, Louisiana)
Ormonde (Madison County, New York)
Ormsby-Kelly House (Palo Alto County, Iowa)
Ormsby-Rosser House (Carson City, Nevada)
Orna Villa (Newton County, Georgia)
Orndoff-Cross House (Berkeley County, West Virginia)
Historic Home near Martinsburg, WV
Orne, Sarah, House (Middlesex County, Massachusetts)
Orrell House (Baldwin County, Alabama)
Orrin B. Hartley House (Hood River County, Oregon)
Orrin W. and Henrietta H. Robertson House (Milwaukee County, Wisconsin)
Lakefront house modeled after Chateau d'Azey-le-Rideau
Orrin W. Burritt House (Cayuga County, New York)
Orrin White House (Washtenaw County, Michigan)
Orrock, Rev. John, House (Norfolk County, Massachusetts)
Orson Ames House (Oswego County, New York)
Orson Everitt House (Wayne County, Michigan)
Orson Grimmett Bungalow (Bear Lake County, Idaho)
Orson Pratt House (Washington County, Utah)
Orson Rogers House (McHenry County, Illinois)
Orth C. Galloway House (Monroe County, Arkansas)
Orth House (Cook County, Illinois)
Ortiz House (Yuma County, Arizona)
Orton Plantation (Brunswick County, North Carolina)
Orton--Malstrom House (Salt Lake County, Utah)
Orville Jackson House (Ada County, Idaho)
Orville P. and Sarah Chubb House (Martin County, Minnesota)
Orville Waring T., House (Union County, New Jersey)
Ory House (East Baton Rouge Parish, Louisiana)
Osborn House (Bristol County, Massachusetts)
Osborn--Bouton--Mead House (Westchester County, New York)
Osborne House (Ontario County, New York)
Osborne Wells House (Newberry County, South Carolina)
Osborne, John, House (Fairfield County, Connecticut)
Osborne, Prince, House (Essex County, Massachusetts)
Osbornedale (New Haven County, Connecticut)
Osburn House (Scott County, Kentucky)
Oscar Anderson House (Anchorage, Alaska)
Oscar B. Jacobson House (Cleveland County, Oklahoma)
Oscar Blomeen House (King County, Washington)
Oscar C. Diehl House (Midland County, Michigan)