| Oakes | Middle English | The dweller by the Oak trees. |
| Oakley | Middle English | One who lives at the Oak tree meadow. |
| Obadiah | Hebrew | The servant of God. |
| Obelix | Greek | pillar of strength |
| Oberon | A character in William Shakespeare's A Midsummer Night's Dream. | |
| Obert | Teutonic | Wealthy and bright. |
| Obiajulu | African | the heart is consoled |
| Ochen | Ugandan | one of the twins |
| Octavius | Latin | The eighth born. |
| Oddvar | Norse | the spear?s point |
| Odell | Old Norse | Wealthy. |
| Odern | Aboriginal | By the sea. |
| Odil | French | rich |
| Odin | Old Norse | The Scandinavian God of war. |
| Odion | Nigerian | first of twins |
| Odolf | Teutonic | A noble wolf. |
| Odon | Hungarian | wealthy protector |
| Odysseus | Greek | full of wrath |
| Ogden | Old English | From the valley of Oak trees. |
| Ogilvie | Celtic | From the high hill. |
| Ogilvy | Old Scottish | from the high peak |
| Oglesby | Old English | Awe-inspiring. |
| Oguz | Hungarian | arrow |
| Okan | Turkish | |
| Okely | Middle English | One who lives at the Oak tree meadow. |
| Okko | Finnish | From the name Oscar. |
| Olaf | Old Norse/Scandinavian | Ancestor. |
| Olcay | Turkish | |
| Oldrich | Czech | one with riches and power |
| Oleg | Russian | The name of an early prince of Kiev. |
| Oleos | Spanish | holy oil used in church |
| Olier | Breton | |
| Olin | Old English | holly |
| Oliver | Latin | An Olive tree or branch. A symbol of peace. |
| Olivier | From Shakespeare's play As You Like It. | |
| Ollie | An Olive tree or branch. A symbol of peace. | |
| Omar | Arabic | First born son. |
| Omarjeet | Hindu | |
| Ompoly | Hungarian | Ampleumian |
| Ond | Hungarian | tenth child |
| Onslow | Old English | The hill of the zealous one. |
| Onur | Turkish | |
| Ora | Latin/Polynesian | Latin: Light, golden. Polynesian: Life. A boy or girl's name. |
| Orad | Aboriginal | Earth. |
| Oram | Old English | From the enclosure by the riverbank. |
| Oran | Irish Gaelic | Pale-skinned. |
| Orazio | Italian | From a Roman family name. |
| Orban | Hungarian | A city dweller. |
| Ordway | Old English | The spear fighter. |
| Orelious | Orelius | |
| Oren | Hebrew | A tree. Also see Oran. |
| Orestes | Greek | A man of the mountain. A hero of Greek mythology. |
| Orfeo | Italian | A name from ancient mythology. |
| Orford | Old English | A dweller at the ford. |
| Orion | Greek | The son of light. The name of a constellation. |
| Orlan | Old English | From the pointed land. A boy or girl's name. |
| Orlando | Italian | Italian form of Roland. Famed throughout the land. The name of Shakespeare's hero in As You Like It. |
| Orman | Old English | spearman |
| Ormond | Old English | A spearman. |
| Ormos | Hungarian | like a cliff |
| Oroiti | Polynesian | The slow-footed one. |
| Orpheus | Greek | A name from ancient mythology. |
| Orran | Irish Gaelic | Pale-skinned. |
| Orren | Irish Gaelic | Pale-skinned. |
| Orrin | Irish Gaelic | Pale-skinned. |
| Orsen | Old French from Latin | A little bear. |
| Orsino | A little bear. | |
| Orson | Old French from Latin | A little bear. |
| Orton | Old English | From the farm by the river. |
| Orville | Old French | Golden city. |
| Orvin | Old English | A spear friend. |
| Osaze | Hebrew | favoured by God |
| Osbert | Old English | Divinely bright or famous. |
| Osborn | Old English | A divine warrior. |
| Osborne | Hebrew | soldier of God |
| Oscar | Old English | A divine spearman. |
| Osgood | Old Norse | A pagan god. |
| Osip | Russian | God shall add. |
| Oskar | German | A divine spearman. |
| Oskari | Finnish | A divine spearman. |
| Osman | Arabic | An Ottoman Turk, or a servant of God. |
| Osmar | Old English | Divinely glorious. |
| Osmond | Old English | A divine protector. |
| Osric | Old English | A divine ruler. |
| Ossian | Latin | fawn |
| Oswald | Old English | Divinely powerful. |
| Oswin | Old English | A friend of God, or a divine friend. |
| Oszlar | Hungarian | |
| Otello | Italian | Italian forms of Otto. Rich, prosperous. |
| Othello | Italian | Prosperous. |
| Otis | Greek | Keen of hearing. |
| Ottavio | Italian | The eighth born. |
| Otto | Teutonic | Rich, prosperous. |
| Otway | Teutonic | Fortunate in battle. |
| Ove | Scandinavian | A well-known Scandinavian name. |
| Ovid | Latin | lamb |
| Owen | Welsh | Well-born. Also a Welsh form of John (see Evan). |
| Oxford | Old English | From the ford of the oxen. An English university city. |
| Oxton | Old English | From the ox enclosure. |
| Oz | Hebrew | strength |
| Ozan | Turkish | |
| Ozaner | Turkish | |
| Ozias | Old English | From the ox enclosure. |
| Ozor | Hungarian | name of an ethnic group |
| Ozsvot | Hungarian | deity, might |