O

obeisance (noun)
Deferential respect.  A gesture expressing deferential respect, such as a bow or curtsy. (OD)
A movement of the body made in token of respect or submission; bow.  Acknowledgment of another's superiority or importance; homage. (MW)
obeisant (adjective)
obeisantly (adverb)

obelus (noun)
The symbol ÷

obstreperous (adjective)
Noisy and difficult to control. (OD)
Marked by unruly or aggressive noisiness; clamorous.  Stubbornly resistant to control; unruly. (MW)

ochlocracy (noun)
Government by the populace; mob rule. (OD)

octothorpe or octothorp (noun)
The symbol #

odd pricing (business practice)
Pricing that does not end in a zero or a number of zeros.  For instance, $1.99.  Conventional wisdom and numerous studies suggest it works because people read left to right.  Therefore, the left-most figure is the most important in terms of perception. (RM)

oenology (noun)
The study of wines. (OD)
oenologist:  an expert in or connoisseur of wines.
oenophile:  a lover or connoisseur of wine.
(US also: enology)

oeuvre (noun)
The body of work of a painter, composer, or author. (OD)

oikophobia (noun)
Psychiatry:  An aversion to home surroundings. (Wikipedia)
Political:  The repudiation of inheritance and home; the antithesis of xenophobia; political ideologies that repudiate one’s own culture and laud others. (Wikipedia)
Roger Scruton on oikophobia:  The disposition, in any conflict, to side with them against us, and the felt need to denigrate the customs, culture and institutions that are identifiably ours.  Being the opposite of xenophobia I propose to call this state of mind oikophobia, by which I mean (stretching the Greek a little) the repudiation of inheritance and home…A chronic form of oikophobia has spread through the American universities, in the guise of political correctness, and loudly surfaced in the aftermath of September 11th, to pour scorn on the culture that allegedly provoked the attacks, and to side by implication with the terrorists…The oik is, in his own eyes, a defender of enlightened universalism against local chauvinism.
(Roger Scruton, England and the Need for Nations, 2006.  Also see James Taranto’s WSJ article, Oikophobia, from 2010.)

omniana (noun)
Thoughts or scraps of information about all or many kinds of things, especially (a collection of) notes, jottings, or short pieces of writing on all or many kinds of subjects. (OED)
Note:  Like a weblog.

ontology (noun)
The part of philosophy that studies what it means to exist. (CD)
ontological argument:  An argument for the existence of God based upon the meaning of the term God. (MW)

opprobrium (noun)
Harsh criticism or censure. (OD)
opprobrious (adjective):  Of language:  Expressing scorn or criticism. (OD)

organization man (noun)
A man who subordinates his individuality and his personal life to the organization for which he works. (OED)
A man who lets his individuality and personal life be dominated by the organization he serves. (OD)
A man who subordinates individualism to conformity with the standards and requirements of an organization. (MW)

orthography (noun)
The conventional spelling system of a language.  The study of spelling and how letters combine to represent sounds and form words. (OD)

Oxbridge (noun and adjective)
The universities of Oxford and Cambridge regarded together.  Of, relating to, characteristic of, or reminiscent of Oxbridge (frequently with implication of superior social or intellectual status). (OED)

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N <=> P