O'Bier House (Webster Parish, Louisiana)
O'Blenis House (Bergen County, New Jersey)
O'Connor House (Pembina County, North Dakota)
O'Dea House (Prince George's County, Maryland)
O'Dell House (Cocke County, Tennessee)
O'Donnell Building (Vanderburgh County, Indiana)
O'Donnell House (Sumter County, South Carolina)
O'Donohue, C. A., House (Suffolk County, New York)
O'Hara House (Ravalli County, Montana)
O'Kane--Jacobs House (Franklin County, Arkansas)
O'Shea House (Carbon County, Montana)
O. A. E. Blyberg House (Otter Tail County, Minnesota)
O. Arthur Kirkman House and Outbuildings (Guilford County, North Carolina)
O. B. McFadden House (Lewis County, Washington)
O. B. Sims House (Ellis County, Texas)
O. C. Barber Creamery (Summit County, Ohio)
O. C. Sarles House (Traill County, North Dakota)
O. D. and Ada Dillingham House (Taylor County, Texas)
O. E. Payne House (Lucas County, Iowa)
O. E. Rolvaag House (Rice County, Minnesota)
O. E. Walker House (Mohave County, Arizona)
O. F. and Mary Prigden House (DeWitt County, Texas)
O. F. Short House (Ada County, Idaho)
O. J. Daniels House (Jerome County, Idaho)
O. K. Palmer House (Lewis County, Washington)
O. L. and Josephine Morgan House (Jefferson County, Washington)
O. L. Dunaway House (Faulkner County, Arkansas)
O. L. Price House (Multnomah County, Oregon)
O. P. Wickham House (Pottawattamie County, Iowa)
O. T. Baker House (Lorain County, Ohio)
O. W. Getchell House (Mendocino County, California)
O.B. Grant House (Elk County, Pennsylvania)
O.F. and Lulu E. Fryer House (Jefferson County, Iowa)
O.J. Gendron Ranch (Yakima County, Washington)
O.W. Francis House (Jewell County, Kansas)
Oak Alley Plantation (St. James Parish, Louisiana)
Oak Crest (Isle of Wight County, Virginia)
Oak Dell (Morris County, New Jersey)
Oak Forest (Hanover County, Virginia)
Oak Glen (Newport County, Rhode Island)
Oak Grove (Cumberland County, North Carolina)
Oak Grove (Hampton County, South Carolina)
Oak Grove (Jefferson County, Mississippi)
Oak Grove (Charles County, Maryland)
Oak Grove (Northampton County, Virginia)
Oak Grove (Campbell County, Virginia)
Oak Hill (Fairfax County, Virginia)
Oak Hill (Loudoun County, Virginia)
Oak Hill (Ross County, Ohio)
Oak Hill (Fauquier County, Virginia)
Oak Hill (Marshall County, Kentucky)
Oak Hill (Colonial Heights, Virginia)
Oak Hill (Columbia County, New York)
Oak Hill (Cumberland County, Virginia)
Oak Hill Cottage (Richland County, Ohio)
Oak Hill Estate (Fayette County, Pennsylvania)
Oak Island (Charleston County, South Carolina)
Oak Knoll (Middlesex County, Massachusetts)
Oak Lawn (Amherst County, Virginia)
Oak Lawn (Caroline County, Maryland)
Oak Top (Montgomery County, Tennessee)
Oak Tree Cottage (Calhoun County, Alabama)
Oakcliff (Saratoga County, New York)
Oakdale (Chester County, Pennsylvania)
Oakdale Cotton Mill Village (Guilford County, North Carolina)
Oakdene (Staunton, Virginia)
Oakdene Place (Floyd County, Georgia)
Oakes Estate (Essex County, New Jersey)
Oakes, Edward, House (Middlesex County, Massachusetts)
Oakes, J. J., House (Worcester County, Massachusetts)
Oakes-Wood House (Johnson County, Iowa)
Oakforest (Wake County, North Carolina)
Oakham (Warren County, Tennessee)
Oakhill (Calhoun County, Michigan)
Oakhurst (Sumter County, Alabama)
Oakhurst Links (Greenbrier County, West Virginia)
Oaklana (Bertie County, North Carolina)
Oakland (Halifax County, North Carolina)
Oakland (Hanover County, Virginia)
Oakland (Adams County, Mississippi)
Oakland (Wood County, West Virginia)
Oakland (Meriwether County, Georgia)
Oakland (Weakley County, Tennessee)
Oakland (Charles County, Maryland)
Oakland (Adams County, Mississippi)
Oakland (Sumner County, Tennessee)
Oakland Plantation (Bladen County, North Carolina)
Oakland Plantation (Calhoun County, South Carolina)
Oakland Plantation (Edgecombe County, North Carolina)
Oakland Plantation (Aiken County, South Carolina)
Oakland Plantation House (Charleston County, South Carolina)
Oakland Plantation House (East Feliciana Parish, Louisiana)
Oakland/Henry House (Christian County, Kentucky)
Oaklands (Rutherford County, Tennessee)
Oaklands (Kennebec County, Maine)
Oaklands (Chester County, Pennsylvania)
Oaklands, The (Kalamazoo County, Michigan)
Oaklawn Manor (St. Mary Parish, Louisiana)
Oaklawn Plantation (Natchitoches Parish, Louisiana)
Oakleigh (Mobile County, Alabama)
Oakley (Spotsylvania County, Virginia)
Oakley (Fauquier County, Virginia)
Oakley (Sumner County, Tennessee)
Oakley (Northumberland County, Virginia)
Oakley Hill (Hanover County, Virginia)
Oakley Plantation House (West Feliciana Parish, Louisiana)
Oakley, John, House (Suffolk County, New York)
Oaklyn Plantation (Darlington County, South Carolina)
Oakmont (Pitt County, North Carolina)
Oakmoor (Hopkins County, Kentucky)
Oaks Cloister (Philadelphia County, Pennsylvania)
Oaks II (Montgomery County, Maryland)
Oaks, The (Hinds County, Mississippi)
Oaks, The (Los Angeles County, California)
Oaks, The (West Feliciana Parish, Louisiana)
Oaks, The (Staunton, Virginia)
Oaks, The (De Soto Parish, Louisiana)
Oakslea Place (Madison County, Tennessee)
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)
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)
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)
Olaf W. Mattson Home (Park County, Montana)
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)