Random Linguistics Definitions Beginning with the Letter "O"

obligatory (Element) that cannot be deleted from a syntactic or other structure. E.g. in She left quickly, neither the subject (She) nor the verb (left) can be removed from the construction: cf. She quickly, Left quickly. Therefore both are obligatory. But She left is complete without quickly: therefore the adverb is optional.

Oghams Alphabet attested by inscriptions in the British Isles from the centuries after the collapse of the Roman empire. The letters are formed by groups of one or more lines inscribed horizontally to the left or right of, or diagonally across, a vertical line or the edge of e.g. a stone. The precise origins both of the system and of its name are uncertain.

Definitions from: The Concise Oxford Dictionary of Linguistics, Second edition, 2007
Image credits: symbol and alphabet.