O (from O to O)
O2 (from O2EM to O2EM)
O_ (from O' Connor Airlines to O Yuki Conjugate)
Oa (from Oa to Oaxaca (state))
Ob (from Ob River to Obwalden)
Oc (from OC-135 Open Skies to Ocyrhoe)
Od (from Od to Odyssey (album))
Oe (from OE to Oeung Choeun)
Of (from Of Mice and Men to Ofwat)
Og (from Oga to Ogyruan)
Oh (from OH-58 Kiowa to Ohrekreis)
Oi (from Oi to Oita prefecture)
Oj (from OJ to Ojus, Florida)
Ok (from OK Computer to OKW)
Ol (from OL to Olyphant, Pennsylvania)
Om (from Om to OMW Familia)
On (from On base percentage to Onyx, California)
Oo (from OO to Oozaru)
Op (from Op art to Opwijk)
Oq (from Oquawka, Illinois to Oquirrh, Utah)
Or (from Or to Oryx)
Os (from Os to Osyka, Mississippi)
Ot (from Ota to Otwock)
Ou (from OU to Ouzo)
Ov (from OV-10 Bronco to Ovum)
Ow (from Owain Gwynedd to Owyhee County, Idaho)
Ox (from OX-5 to Oxyurinae)
Oy (from Oya to Oystercatcher)
Oz (from Oz to Ozzy Osbourne)