முன்னொட்டுச் சுட்டி
- O'odham
- OBC
- OB association
- OC3
- OECD
- OEF
- OESC
- OFC
- OFF
- OGA
- OGTT
- OHQ
- OIE
- OIH
- OIOS
- OK
- OLA
- ONGC
- ONU
- ONUCI
- OO
- OOTB
- OPM
- OPML
- OPPF
- OPS
- OR gate
- OSCE
- OSF
- OTCD
- OTH
- OTPROM
- OUO
- OYW
- Oasis
- Objects
- Obstruction Rule
- Occam's razor
- Occidental
- Occitan
- Oceanic whitetip shark
- Octaves
- Ogos
- Ohlone
- Ohm
- Ohm's law
- Oireachtas
- Ojibwe
- Okarito brown kiwi
- Okazaki fragment
- Okinawan
- Old Armenian
- Old Breton
- Old Church Slavonic
- Old East Slavic
- Old English
- Old French
- Old Frisian
- Old Glory
- Old High German
- Old Irish
- Old Japanese
- Old Norse
- Old Occitan
- Old Persian
- Old Portuguese
- Old Provenal
- Old Prussian
- Old Saxon
- Old Slavonic
- Old Spanish
- Old Testament
- Old Tupi
- Old Turkic
- Old World knifefish
- Old World rivuline
- Oligochaeta
- Olive ridley sea turtle
- Olympiadane
- Oman
- Omnes determinatio cst negatio
- Ongole
- Online class
- Oparin's theory
- Opel.John
- Open University
- Open surgery
- Opening entry
- Opening stock
- Ophiuchus
- Ops
- Oracle
- Oral Polio Vaccine
- Oral glucose tolerance test
- Orang-outang
- Orange lily
- Ord-inCouncil
- Order-in-Council
- Ordnance Survey
- Ordovician period
- Oregon chub
- Oriental dollarbird
- Oriental loach
- Orientalism
- Orientalist
- Original Macintosh Keyboard
- Orion
- Oriya
- Oromo
- Orpington
- Orson
- Oscan
- Oshindonga
- Osroene
- Ossetian
- Ossetic
- Ostrowiec Świętokrzyski
- Ostwald-Walker method
- Ostwald process
- Ostwald theory
- Otaheite apple
- Otomi
- Ototoxicity
- Owens pupfish
- Own Basket