O

O curas hominum! O quantum est in rebus inane! - Ah, human cares! Ah, how much futility in the world! (Lucilius)
O di immortales! - Good heavens! (uttered by Cicero on the Senate floor)
O diem praeclarum! - Oh, what a beautiful day!
O praeclarum custodem ovium lupum! - An excellent protector of sheep, the wolf! (Cicero)
O quam cito transit gloria mundi! - O how quickly passes the glory of the world!
O sancta simplicitas! - Oh, holy simplicity! (Jan Hus)
O tempora, O mores! - Oh, the times! Oh, the morals! (Cicero)
O! Plus! Perge! Aio! Hui! Hem! - Oh! More! Go on! Yes! Ooh! Ummm!
Obesa cantavit - The fat lady has sung
Obiit (ob.) - He/she died
Obiter (ob.) - In passing
Obiter dictum - Something said in passing - parenthetical remark
Oblitus sum perpolire clepsydras! - I forgot to polish the clocks!
Obscurum per obscurius - The obscure by means of the more obscure
Obsta principiis - Resist the beginnings - Nip it in the bud
Occasio aegre offertur, facile amittitur - Opportunity is offered with difficulty, lost with ease. (Publius Syrus)
Occasio facit furem - Opportunity makes a thief
Oderint dum metuant - Let them hate provided that they fear. (Seneca)
Odi et amo - I hate (her), and I love (her) (Catullus)
Odium theologicum - Theological hatred. (a special name for the hatred generated in theological disputes)
Olevm addere camino - To pour fuel on the stove adding gasoline to a fire
Olevm perdisti - You have lost oil you've wasted your time on this criticism for a misallocation of resources
Olim habeas eorum pecuniam, numquam eam reddis: prima regula quaesitus - Once you have their money, you never give it back: the 1st rule of acquisiton
Olim - Formerly
Omne ignotum pro magnifico est - We have great notions of everything unknown. (Tacitus)
Omne initium est difficile - Every beginning is difficult
Omne trium perfectum - Everything that comes in threes is perfect
Omne tvlit pvnctvm qvi miscvit vtile dvlci - [he] has gained every point who has combined [the] useful [with the] agreeable
Omnes aequo animo parent ubi digni imperant - All men cheerfully obey where worthy men rule. (Syrus)
Omnes deteriores svmvs licentia - Too much freedom debases us
Omnes lagani pistrinae gelate male sapiunt - All frozen pizzas taste lousy
Omnes una manet nox - The same night awaits us all. (Horace)
Omnes vulnerant, ultima necat - All (hours) wound, the last kills. (inscription on solar clocks)
Omnia iam fient quae posse negabam - Everything which I used to say could not happen will happen now. (Ovid)
Omnia mea mecum porto - All that is mine, I carry with me. (My wisdom is my greatest wealth) (Cicero)
Omnia mihi lingua graeca sunt - It's all Greek to me
Omnia mors aequat - Death equals all things
Omnia munda mundis - Everything is pure to pure ones
Omnia mutantur nos et mutamur in illis - All things change, and we change with them
Omnia mutantur, nihil interit - Everything changes, nothing perishes. (Ovid)
Omnia mutantur, nos et mutamur in illis - All things are changing, and we are changing with them
Omnia vincit amor - Love conquers all
Omnia vincit amor; et nos cedamus amori - Love conquers all things; let us too surrender to love. (Vergil)
Omnium gatherum - Assortment
Omnium rerum principia parva sunt - Everything has a small beginning. (Cicero)
Onus probandi - The burden of proof
Opere citato (op. cit.) - In the work just quoted
Optimis parentibus - To my excellent parents. A common dedication in a book
Optimus magister, bonus liber - The best teacher is a good book
Opus Dei - The work of God
Ora et labora - Pray and work. (St. Benedict)
Ora pro nobis - Pray for us
Oratvr fit, poeta nascitvr - An orator is made [but] a poet is born
Orbes volantes exstare - Flying saucers are real
Orbiter dictum/dicta - Said by the way (miscellaneous remarks)
Orcae ita - Pretty straightforward
Ore rotundo - With full voice
Osculare pultem meam! - Kiss my grits!