| Latin | Translation | Notes |
| O Deus ego amo te | O God I Love You | attributed to Saint Francis Xavier |
| ' | The farmers would count themselves lucky, if only they knew how good they had it | from Virgil in Georgics II, 458 |
| o homines ad servitutem paratos | Men ready to be slaves! | attributed to the Roman Emperor Tiberius, in disgust at the servile attitude of Roman senators; said of those who should be leaders but instead slavishly follow the lead of others |
| O tempora, o mores! | Oh, the times! Oh, the morals! | also translated "What times! What customs!"; from Cicero, Catilina I, 2 |
| O Tite tute Tati tibi tanta tyranne tulisti | O tyrant Titus Tatius, what terrible calamities you brought onto yourself! | from Quintus Ennius, Annales, considered an example of a Latin tongue-twister |
| Obedientia civium urbis felicitas | The obedience of the citizens makes us a happy city | Motto of Dublin |
| obiit | one died | "He/she died", inscription on gravestones; ob. also sometimes stands for obiter |
| obit anis, abit onus | The old woman dies, the burden is lifted | Arthur Schopenhauer |
| obit caeleps | Ob. Cael. or died a bachelor | Heraldic visitation or County Visitation Books for England |
| obiter dictum | a thing said in passing | in law, an observation by a judge on some point of law not directly relevant to the case before him, and thus neither requiring his decision nor serving as a precedent, but nevertheless of persuasive authority. In general, any comment, remark or observation made in passing |
| obliti privatorum, publica curate | Forget private affairs, take care of public ones | Roman political saying which reminds that common good should be given priority over private matters for any person having a responsibility in the State |
| obscuris vera involvens | the truth being enveloped by obscure things | from Virgil |
| obscurum per obscurius | the obscure by means of the more obscure | An explanation that is less clear than what it tries to explain; synonymous with ignotum per ignotius |
| obtineo et teneo | to obtain and to keep | motto |
| obtorto collo | with a twisted neck | unwillingly |
| oculus dexter | right eye | Ophthalmologist shorthand |
| oculus sinister | left eye | - |
| oderint dum metuant | let them hate, so long as they fear | favorite saying of Caligula, attributed originally to Lucius Accius, Roman tragic poet |
| odi et amo | I hate and I love | opening of Catullus 85; the entire poem reads, "odi et amo quare id faciam fortasse requiris / nescio sed fieri sentio et excrucior" |
| odi profanum vulgus et arceo | I hate the unholy rabble and keep them away | Horace, Carmina III, 1 |
| odium theologicum | theological hatred | name for the special hatred generated in theological disputes |
| oleum camino | oil on the fire | from Erasmus' collection of annotated Adagia |
| omne ignotum pro magnifico | every unknown thing for great | or "everything unknown appears magnificent" The source is Tacitus: Agricola, Book 1, 30 where the sentence ends with 'est'. The quotation is found in Arthur Conan Doyle's Sherlock Holmes short story "The Red-Headed League" where the 'est' is missing. |
| omne initium difficile est | every beginning is difficult | |
| ' | every living thing is from an egg | foundational concept of modern biology, opposing the theory of spontaneous generation |
| Omnes homines sunt asini vel homines et asini sunt asini | All men are donkeys or men and donkeys are donkeys | a sophisma proposed and solved by Albert of Saxony |
| omnes vulnerant, postuma necat, or, omnes feriunt, ultima necat | all wound, last one kills | usual in clocks, reminding the reader of death |
| omnia cum deo | all with God | motto for Mount Lilydale Mercy College, Lilydale, Victoria, Australia |
| omnia dicta fortiora si dicta Latina | everything said stronger if said in Latin | or "everything sounds more impressive when said in Latin"; a more common phrase with the same meaning is quidquid Latine dictum sit altum videtur |
| omnia in mensura et numero et pondere disposuisti | Thou hast ordered all things in measure, and number, and weight. | Book of Wisdom, 11:21 |
| Omnia mea mecum porto | All that is mine I carry with me | is a quote that Cicero ascribes to Bias of Priene |
| omnia mutantur, nihil interit | everything changes, nothing perishes | Ovid, Metamorphoses, book XV, line 165 |
| omnia omnibus | all things to all men | 1 Corinthians 9:22 |
| si omnia ficta | if all is fiction | Ovid, Metamorphoses, book XIII, lines 733–4: "si non omnia vates ficta" |
| omnia vincit amor | love conquers all | Virgil, Eclogue X, line 69 |
| omnia munda mundis | everything pure to the pure | from The New Testament |
| omnia praesumuntur legitime facta donec probetur in contrarium | all things are presumed to be lawfully done, until it is shown in the reverse | in other words, "innocent until proven guilty" |
| omnia sponte fluant absit violentia rebus | everything should flow by itself, force should be absent | "let it go" |
| omnia sunt communia | all things shall be held in common | from Acts of the Apostles |
| omnis vir enim sui | Every man for himself! | |
| omnibus idem | the same to all | motto of Pieter Corneliszoon Hooft, usually accompanied by a sun, which shines for everyone |
| omnibus locis fit caedes | There is slaughter everywhere | Julius Caesar's The Gallic War, 7.67 |
| omnis traductor traditor | every translator is a traitor | every translation is a corruption of the original; the reader should take heed of unavoidable imperfections |
| omnis vir tigris | everyone a tiger | motto of the 102nd Intelligence Wing |
| omnium gatherum | gathering of all | miscellaneous collection or assortment; "gatherum" is English, and the term is used often used facetiously |
| onus probandi | burden of proof | |
| onus procedendi | burden of procedure | burden of a party to adduce evidence that a case is an exception to the rule |
| opera omnia | all works | collected works of an author |
| opera posthuma | posthumous works | works published after the author's death |
| operari sequitur esse | act of doing something follows the act of being | scholastic phrase, used to explain that there is no possible act if there is not being: being is absolutely necessary for any other act |
| opere citato | in the work that was cited | used in academic works when referring again to the last source mentioned or used |
| opere et veritate | in action and truth | doing what you believe is morally right through everyday actions |
| opere laudato | | See opere citato |
| operibus anteire | leading the way with deeds | to speak with actions instead of words |
| ophidia in herba | a snake in the grass | any hidden danger or unknown risk |
| opinio juris sive necessitatis | an opinion of law or necessity | a belief that an action was undertaken because it was a legal necessity; source of customary law |
| opus anglicanum | English work | fine embroidery, especially used to describe church vestments |
| Opus Dei | The Work of God | Catholic organisation |
| ora et labora | pray and work | This principle of the Benedictine monasteries reads in full: "Ora et labora, Deus adest sine mora." "Pray and work, God is there without delay" |
| ora pro nobis | pray for us | "Sancta Maria, mater Dei, ora pro nobis peccatoribus"; Brazilian name for Pereskia aculeata |
| orando laborando | by praying, by working | motto of Rugby School |
| oratio recta | direct speech | expressions from Latin grammar |
| oratio obliqua | indirect speech | expressions from Latin grammar |
| ' | speech for house | also abbreviated pro domo; speak on one's own behalf; based on a speech by Cicero in legal proceedings in 57 AD to regain his house on the Palatine Hill that was confiscated during his exile |
| ' | the world does not suffice or the world is not enough | from Satires of Juvenal, referring to Alexander the Great; James Bond's adopted family motto in the novel On Her Majesty's Secret Service; it made a brief appearance in the film adaptation of the same name and was later used as the title of the nineteenth James Bond film, The World Is Not Enough. |
| ' | one world | seen in The Legend of Zorro |
| ' | out of chaos, comes order | one of the oldest mottos of Craft Freemasonry. |
| pro invicem | , one for the other; let us pray for each other | Popular salutation for Roman Catholic clergy at the beginning or ending of a letter or note. Usually abbreviated OPI.. |
| newly risen, how brightly you shine | Motto of New South Wales |