O
From Encyclopediak
- Oak
- Oakland, California
- Oats
- 0badiah
- Obelisk
- Oberammergau
- Oberlin College
- Obi River
- Oboe
- Obregon, Alvaro
- Observatory
- Obsidian
- Ocean
- Ocean Grove, New Jersey
- Ocelot
- Ocher
- Ocmulgee, River
- Oconee, River
- O'Connell, Daniel
- O'Connor, Thomas Power
- Octave, Thanet
- Octavia
- October
- Octopus
- Odd Fellows, Independent Order of
- Ode
- Oder River
- Odessa, Russia
- Odin
- Odoacer
- Odyssey
- Oedipus
- Offenbach, Jacques
- Ogden, Utah
- Ogdensburg, New York
- Oglesby, Richard James
- Oglethorpe, James Edward
- O'Hagan, Thomas
- 0'Henry
- Ohio
- Ohio and Erie Canal
- Ohio River
- Ohio, State Universities of
- Ohm
- Ohm, Georg Simon
- Ohm's Law
- Oil City, Pennsylvania
- Oilcloth
- Oils
- Ojibway
- Okapi
- Okechobee Lake
- Okhotsk, Sea of
- Oklahoma
- Oklahoma City, Oklahoma
- Oklahoma, University of
- Oldenburg, Germany
- Old Forge, Pennsylvania
- Old Ironsides
- Old South Church
- Olean, New York
- Oleander
- Olefiant Gas
- Oleomargarine
- Oliphant, Mrs. Margaret
- Olive
- Olive Oil
- Oliver Optic
- Olives, Mount of
- Olmsted, Frederick Law
- Olney, Richard
- Olympia, Washington
- Olympian Games
- Olympus
- Olyphant, Pennsylvania
- Omaha Indians
- Omaha, Nebraska
- Oman
- Omar Khayyam
- Omnibus
- Omnibus Bill
- Omsk, Russia
- Onega, Lake
- Oneida Castle
- Oneida Indians
- Oneida, New York
- Oneonta, New York
- Onion
- Onondaga Indians
- Ontario Canada
- Onyx
- Onyx Marble
- Opal
- Open-Door Policy
- Open-Hearth, Furnace
- Opera
- Opera Glass
- Ophthalmoscope
- Opium
- Oporto, Portugal
- Opossum
- Opper, Frederick Burr
- Optic, Oliver
- Optics
- Optimism
- Oracles
- Orange
- Orange, New Jersey
- Orangemen
- Orange River
- Orange River Colony
- Orange Free State
- Orangutan
- Orbit
- Orcagna, Andrea
- Orchid
- Ord, Edward Otho Cresap
- Ordeal, Trial by
- Order
- Orders in Council
- Ordinance of 1787
- Oregon
- Oregon, The
- Oregon. University of
- O'Reilly, Charles
- O'Reilly, John Boyle
- O'Reir, Max
- Orestes
- Organ
- Oriental Art
- Origen
- Orinoco River
- Oriole
- Orion
- Oriskany, Battle of
- Orkney Islands
- Orleans
- Orleans, Maid of
- Ornithorogy
- Ornithorhynchus
- Orontes
- Orpheus
- Orthoceras
- Orthoclase
- Orthoptera
- Ortolan
- Osage
- Osage, River
- Osaka, Japan
- Oscar II
- Osceola
- Oshkosh, Wisconsin
- Osier
- Osiris
- Oskaloosa, Iowa
- Osler, Sir William
- Osmium
- Osmosis
- Osprey
- Ossian
- Ossining, New York
- Ossoli, Marchioness
- Ostend Manifesto
- Osteopathy
- Ostrich
- Ostrogoths
- Oswego, New York
- Oswego, Battle of
- Otis, Elwell Stephen
- Otis, James
- Otranto, Strait of
- Ottawa Indians
- Ottawa, Canada
- Ottawa River
- Otter
- Otter, William Dillon
- Otto I
- Ottoman Empire
- Ottumwa, Iowa
- Otway, Thomas
- Ouida
- Ounce Cat
- Ounce
- Ouzel
- Ovenbird
- Ovid
- Owen, Robert
- Owen Meredith
- Owensboro, Kentucky
- Owen Sound, Ontario, Canada
- Owl
- Owosso, Michigan
- Oxalic Acid
- Oxenstierna, Axel
- Oxford, England
- Oxford Movement
- Oxford, University of
- Oxygen
- Oyama, Iwao, Prince
- Oyster
- Oyster Bay, New York
- Oyster Catcher
