Oak Creek Fort on, Texas – Fort Chadbourne.
Oak Island Fort on, North Carolina – Fort Caswell.
Oakland Fort, Florida – Five miles from Fort Doane.
Oak Point, Fort, North Carolina – At Wilmington.
Oblong Redoubt, New York – Near West Point.
O’Brien Fort, Maine.
Observation Camp, Maryland – Near Poolesville.
Ocilla, Fort, Florida – Between Ocilla and Wacissa rivers.
Ocklawaha Fort, Florida – Near Apalachicola.
Ocklockony River Fort on, Florida – Thirty miles from the sea.
Ockracoke Inlet Fort at, North Carolina – Fort Morgan.
Ocmulgee Fort, Georgia
Ogden Fort, Florida – De Soto County; now town of that name, near Pease River, on Southern Railway.
Ogeechee Fort, Georgia – Near the Ogeechee River.
Oglethorpe (old) Barracks, Georgia – On St. Simons Island.
Oglethorpe Barracks, Georgia – At Savannah.
Oglethorpe, Fort, Georgia- At Savannah.
Ohio Falls Fort at, Indiana – Opposite Louisville, KentucKentucky
Ohio State Soldiers’ Home, Sandusky, Ohio.
Ojo Calienti Camp at, New Mexico – An outpost of Fort Craig.
Okanagon Fort, Washington – On Columbia River, at its junction with the Okanagon.
Olden Camp, New Jersey – At Trenton.
Old Gate General Hospital, Tennessee – At Memphis.
Old Point Comfort, Fort at, Virginia – Fort Monroe.
Old State General Hospital, Tennessee – At Memphis.
Old Tappan Cantonment at, New Jersey.
Old Topsail Inlet Fort, North Carolina -{See Topsail Inlet, old.)
Old Tower Georgia – On Tybee Island.
Olive Bend Creek Stockade, Ohio – Near Waterford.
Olmstead Camp, Virginia – Near Falls Church.
Olmus Camp, Texas – Near San Antonio.
Olney, Stephen, Camp, South Carolina – At Hilton Head.
Olympia Camp, Vermont – At Burlington.
Omaha Barracks or Fort, Nebraska – At Omaha.
O’Meary Lunette, Tennessee – At Chattanooga
One Number Fort, Florida – Eleven miles from Indian River.
One Number Fort, Georgia – Near Savannah.
One Number Battery, Kentucky- Near Island No. 10.
One Number Fort, Massachusetts – On Charles River.
One Number Battery, Virginia – On Wormsley Creek.
Oneida Fort at, New York – Fort Bull.
Onion River Fort on, Vermont.
Onondaga Battery, Virginia.
Ontario Fort, New York- At Oswego, on site of old Fort Pepperell.
Onward Camp, Georgia – At Savannah.
Onyades Fort, New York.
Opelika Fort, Alabama – At Opelika.
Oplandt Fort, New Jersey – Near Lewiston.
Orange (1) Fort, New York – At Bowling Green.
Orange (2) Fort, New York – At Albany.
Ord Camp, Arizona – Now Fort Apache.
Oregon Fort, North Carolina – At Ocracoke Inlet.
Oregon State Soldiers’ Home, Roseburg, Oregon.
Orford Fort, Oregon – At Port Orford.
Orleans Fort, Missouri – Near mouth of the Osage.
O’Rourke Battery, South Carolina – On Morris Island.
O’Rourke Fort, Virginia – A defense of Washington, D. C.
Orr Camp, Pennsylvania – Near Kittanning.
Osage Fort, Missouri – On Missouri River, near present site of Sibley.
Osao Fort, New York – At Auburn.
Osborne Camp, Idaho – At Osburn.
Osceola, Fort, Arkansas – At Plum Point, Mississippi River.
Osceola Battery, Florida – At Key West.
Osoyoos Camp, Washington- On Lake Osoyoos, Okinakane Valley.
Ossabaw Sound Fortifications at, Georgia
Ossippee Pond Fort at, New Hampshire.
Oswegatchie Fort, New York – Near Ogdensburg.
Oswego Fort, New York – Present site of Fort Ontario; originally Fort Pepperell.
Otis Camp, Hawaii – At Honolulu.
Otter Island Fort on, South Carolina – St. Helena Sound, Fort Drayton.
Ouatanon or Oulatenon or Ouachtanon Fort, Indiana – On Wabash River, near Lafayette.
Overalls Creek Blockhouse on, Tennessee – Near Murfreesboro.
Overton General Hospital, Tennessee – At Memphis.
Overton Hill Fortifications on, Tennessee – Five miles south of Nashville.
Owen Fort, Montana – On Bitter Root or St. Marys River.
Owen Fort, Virginia – At Petersburg.
Owyhee River Camp, Idaho – Opposite Fort Boise.
Ox Fort, New York – Near source of Allegheny.
Oxford Fort at, Massachusetts.
Oxford Park Camp at, Pennsylvania – Three miles from Frankford.
Oyster Bay Cantonment at, New York – On Long Island.
Oyster Island Battery, New York – On Ellis Island.
Oyster River, Maine – Fortified houses on.