Commercial building

Northern National Bank (Philadelphia County, Pennsylvania)
Northern Pacific Office Building (Pierce County, Washington)
Northern Pacific Warehouse (Sanders County, Montana)
Northern Saving Fund and Safe Deposit Company (Philadelphia County, Pennsylvania)
Northwestern Improvement Company Store (Kittitas County, Washington)
Northwestern National Bank Building (Multnomah County, Oregon)
Norton--Johnson Buick Company (Oklahoma County, Oklahoma)
Norway Center Store (Lincoln County, South Dakota)
Noxon Bank Building (Saratoga County, New York)
O Street Market (Washington, District of Columbia)
O'Connor--Proctor Building (Victoria County, Texas)
O'Hanlon Building (Forsyth County, North Carolina)
O'Kane Building (Deschutes County, Oregon)
O'Neil Lumber Company Office (Flathead County, Montana)
O'Neil's Department Store (Summit County, Ohio)
O'Neill Brothers Building (Elmore County, Idaho)
O'Rourke's Diner (Middlesex County, Connecticut)
O. E. Woods Lumber Co. Building (Rogers County, Oklahoma)
O. G. Anderson & Co. Store (Lyon County, Minnesota)
O. K. Market (Saunders County, Nebraska)
O. T. Bassett Tower (El Paso County, Texas)
Oakes National Bank Block (Dickey County, North Dakota)
Oakhurst Diner (Dutchess County, New York)
Re-opened as the Oakhurst Diner in 2010.
Oakville Grocery (Napa County, California)
Oasis Drive-In (McLean County, Illinois)
Oasis Lounge (Yavapai County, Arizona)
Oates Building (Polk County, Florida)
Oatman Drug Company Building (Mohave County, Arizona)
Oatman Filling Station (Eau Claire County, Wisconsin)
Occidental Life Building (Bernalillo County, New Mexico)
Occidental Life Insurance Company Building (Wake County, North Carolina)
Ocean House (Westchester County, New York)
A Bixler Manufacturing Co. diner converted to a restaurant.
Odd Fellows Block (Union County, Iowa)
Odd Fellows Building (Tehama County, California)
Odd Fellows Building (Pike County, Kentucky)
Odd Fellows Building (Daviess County, Kentucky)
Odd Fellows Lodge (Wayne County, North Carolina)
Odd Fellows Temple (Fayette County, Kentucky)
Odd Fellows Temple (Columbiana County, Ohio)
Oddfellows Building (Middlesex County, Massachusetts)
Oddfellows House (Fairbanks North Star Borough, Alaska)
Oeldorf Building/Wetherell's Jewelers (Wood County, West Virginia)
Offenhauser Insurance Building (Bowie County, Texas)
Ogilvie Building (Huron County, Michigan)
Ogilvie Hardware Company Building (Caddo Parish, Louisiana)
Ohio Company Land Office (Washington County, Ohio)
Ohio Farm Bureau Federation Offices (Franklin County, Ohio)
Ohio National Bank (Franklin County, Ohio)
Ohio Oil Company Building (Natrona County, Wyoming)
Ohio One--Ohio Edison (Mahoning County, Ohio)
Ohio Power Company (Muskingum County, Ohio)
Ojibwa Courier Press Building (Sawyer County, Wisconsin)
Demolished - collapsed and destroyed
Oklahoma Gas and Electric Company Building (Oklahoma County, Oklahoma)
Oklahoma Hardware Building (Oklahoma County, Oklahoma)
Oklahoma Natural Gas Company Building (Tulsa County, Oklahoma)
Oklahoma Publishing Company Building (Oklahoma County, Oklahoma)
Oklahoma State Bank Building (Logan County, Oklahoma)
Old Atoka State Bank (Atoka County, Oklahoma)
Old Bakery (Travis County, Texas)
Old Bank of America Building (Tehama County, California)
Old Bank of Fayetteville Building (Washington County, Arkansas)
Old Barber Shop (Maricopa County, Arizona)
Old Barnhill Building (Moniteau County, Missouri)
Old Bartlett and Goble Store (Morrow County, Ohio)
Old Bell Telephone Building (Mississippi County, Arkansas)
Old Bittermann Building (Vanderburgh County, Indiana)
Old Brick Store (New Castle County, Delaware)
Old Brick Warehouse (Marion County, South Carolina)
Old Bryant--Link Building (Jones County, Texas)
Old Colony Buildings (Cook County, Illinois)
Old Commercial National Bank Building (Caddo Parish, Louisiana)
Old Continental State Bank (Denton County, Texas)
Old Customshouse (Erie County, Pennsylvania)
Old Deery Inn (Sullivan County, Tennessee)
Old Delta Democrat Times Building (Washington County, Mississippi)
Old Dominion Bank Building (Alexandria, Virginia)
Old Eagle Tavern (Mercer County, New Jersey)
Old Farmers and Merchants State Bank (Jackson County, Mississippi)
Old Federal Reserve Bank (Philadelphia County, Pennsylvania)
Old Fellwock Auto Company (Vanderburgh County, Indiana)
Old First National Bank Building (Marion County, Oregon)
Old First National Bank Building (Beckham County, Oklahoma)
Old German-American State Bank (Shawnee County, Kansas)
Old Grolier Club (New York County, New York)
Old Guaranty Bank Building (Lafayette Parish, Louisiana)
Old Harbour Store (Erie County, Ohio)
Old Houston National Bank (Harris County, Texas)
Old Inn, The (Talbot County, Maryland)
Old Log Cabin Inn (Livingston County, Illinois)
Old Ludlow Cafe (San Bernardino County, California)
Old Main Building (Cook County, Illinois)
Old Market Building (Georgetown County, South Carolina)
Old Meat Market-Old Meat Market (Orleans Parish, Louisiana)
Old Mercantile Building (Sutton County, Texas)
Old Morrison Bank (Noble County, Oklahoma)
Red Sandstone Romanesque Bank Building
Old Napa Register Building (Napa County, California)
Old Nebraska State Bank Building (Holt County, Nebraska)
Old New England Building (Jackson County, Missouri)
Old New York Evening Post Building (New York County, New York)
Old Penick--Hughes Company (Jones County, Texas)
Old Petaluma Opera House (Sonoma County, California)
Old Rock Saloon (Hill County, Texas)
Old Rose Tree Tavern (Delaware County, Pennsylvania)
Old Second National Bank (Kane County, Illinois)
Old Slave Mart (Charleston County, South Carolina)
Old South Restaurant (Pope County, Arkansas)
Old State Bank (Knox County, Indiana)
Old State Bank (Floyd County, Indiana)
Branch of the State Bank
Old State Mutual Building (Worcester County, Massachusetts)
Old Stone Blacksmith Shop (Addison County, Vermont)
Old Stone House (Malheur County, Oregon)
Old Stone Tavern (Franklin County, Kentucky)
Old Stone Warehouse (Monroe County, New York)
Old Tavern (Sacramento County, California)
Old Town Savings Bank (Baltimore, Maryland)
Old West Texas Utilities Company (Jones County, Texas)
Old Wheeler-Kelly-Hagny Building (Sedgwick County, Kansas)
Olde Store (John O'Ferrell Store) (Lagrange County, Indiana)
Olds, Wortman and King Department Store (Multnomah County, Oregon)
Oleson-Crane Building (Lincoln County, Oklahoma)
Olinsky Building (Mendocino County, California)
Oliver Bair H., Funeral Home (Philadelphia County, Pennsylvania)
Oliver Building (Cook County, Illinois)
Oliver Springs Banking Company (Anderson County, Tennessee)
Olivier Store (Iberia Parish, Louisiana)
Oljato Trading Post (San Juan County, Utah)
Olsen and Weygandt Building (Multnomah County, Oregon)
Olympia Diner (Hartford County, Connecticut)
Moved here from Massachusetts in 1954, it is possily the largest O'Mahony ever built.
Olympia National Bank (Thurston County, Washington)
Olympic Club (Island County, Washington)
Olympic Club Saloon (Lewis County, Washington)
Omaha Bolt, Nut and Screw Building (Douglas County, Nebraska)
Omaha National Bank Building (Douglas County, Nebraska)
One Charles Center (Baltimore, Maryland)
One LaSalle Street Building (Cook County, Illinois)
One Thirty North Eighth Building (Adams County, Illinois)
Ong Yut Geong Wholesale Market (Maricopa County, Arizona)
Onondaga County Savings Bank Building (Onondaga County, New York)
Ontario State Bank Block (San Bernardino County, California)
Opa-locka Bank (Miami-Dade County, Florida)
Opal's Steak House (Garland County, Arkansas)
Opera Block House (Goodhue County, Minnesota)
Opera Hall Block (Gentry County, Missouri)
Opera House (Webster County, Nebraska)
Opera House and Yates Bookshop Building (Fayette County, Kentucky)
Opera House Block/Central Block Building (Larimer County, Colorado)
Opera House-Starnes Jewelers Building (Stanly County, North Carolina)
Optima Grain Elevator (Texas County, Oklahoma)
Oregon Bank Building (Klamath County, Oregon)
Oregon Portland Cement Building (Multnomah County, Oregon)
Oregon State Bank Building (Multnomah County, Oregon)
Oriental Building Association No. 6 Building (Washington, District of Columbia)
Orme Wilson and Company Storehouse (Loudon County, Tennessee)
Oroville Commercial District (old) (Butte County, California)
Osage Bank of Fairfax (Osage County, Oklahoma)
Oscar Easley Block (Orange County, California)
Oscar Redman Building (Searcy County, Arkansas)
Osceola Times Building (Mississippi County, Arkansas)
Osler Building (Oklahoma County, Oklahoma)
Osterman and Tremaine Building (Dodge County, Nebraska)
Osterman's Shell Station (Mohave County, Arizona)
Otero's 66 Service (Valencia County, New Mexico)
Otis Elevator Company Building (Multnomah County, Oregon)
Ott's Assay Office (Nevada County, California)
Otto Schlemmer Building (Montgomery County, Indiana)
Our House (Gallia County, Ohio)
Overfield Tavern (Miami County, Ohio)
Overland Arizona Co. (Maricopa County, Arizona)
Overton Hygienic Building (Cook County, Illinois)
Ovide LaCour Store (Pointe Coupee Parish, Louisiana)
Owl Blacksmith Shop (Custer County, Oklahoma)
Owl Drug Store (Beckham County, Oklahoma)
P. J. Barrett Block (Berkshire County, Massachusetts)
P.A. Sanning Store (Miller County, Missouri)
Pacific Brewing and Malting Company (Pierce County, Washington)
Pacific Building (Multnomah County, Oregon)
Pacific Desk Building (Bernalillo County, New Mexico)
Pacific Electric Railway Company Substation No. 8 (Los Angeles County, California)
Pacific Express Stable (Denver County, Colorado)
Pacific National Bank Building (Pierce County, Washington)
Lost six-story steel-framed skyscraper
Pacific Press Building (Alameda County, California)
Pacific Telephone and Telegraph Building (Cowlitz County, Washington)