obelisk (†)
obelism
object
object complement
object pronoun
oblique (/)
oblique case
occlusive
OED
officialese
officious | official
of-genitive
Ogden, C.K.
Ogham
Oh | O
Old Church Slavonic
Old English
Old French
Old High German
Old Low German
Old North French
Old Saxon
oligosynthetic language
onomasiology
onomasticon
onomastics
onomatechny
onomatology
onomatopoeia
onomatopoeic coinage
onomatotechny
onset
open-class item
open word class
optative mood
ordinance | ordnance
oronyms
orthoepy
orthographic depth
orthographic principle
orthography
ostensive definition
OV language
Oxford comma
Oxford English Dictionary
oxymoron