oar: Wooden (usually) pole with a blade at the end used to propel a boat. From Old Norse “ar”=oar.

oath: Formal and serious promise, usual invoking a god as witness to the truth. From Old English “eth”=an oath < possibly Greek “oitos”=fate

obacerate: To close ones mouth. Latin “obacerare”=to contradict < “ob”=against + ? “acer”=sharp.

obarmate: To arm against. Latin “obarmare”=to arm < “ob-“=against + “armare”=to arm.

obdurate: Hardened against feeling; hard-hearted. From Latin “obduratus”=hardened < “ob-“=against + “durus”=hard.

obelisk: Tapering, four-sided pillar or column of stone with a pyramidal top. Latin “obelisqus” < Greek “obeliskos”=small spit.

obey: Follow the commands of someone. From Old French “obeir” < Latin “obedire”=to pay attention < “ob-“=towards/to + “audere”=to hear.

obfuscate: To make obscure or difficult to follow. From Latin “obfuscare”=to darken, which comes from “ob”=over + “fuscus”=dark.

obfuscate: To make something unclear, indistinct, or difficult to understand. From Latin “ob-”=over + “fuscare”=to darken. Lit. over-darken.

obituary: List of recorded deaths, often printed in a newspaper. From Latin “obire”=go to meet + “-ary”=connected to. Going to meet death.

oblivious: Not noticing things happening around you. From Latin “oblivisci”=to forget < “ob”=over + Greek “leios”=smooth.

obscure: Dark, dim, or unclear. From Latin “obscurus”=covered over < “ob-“=over + “scurus”=covered. c.f Latin “scutum”=shield.

obsequious: Servile agreement with people simply to please. From Latin “obsequiosus”=compliant < “ob-“=toward + “sequi”=follow + “ous”=full of.

obtrusive: Being excessively or annoyingly self-assertive. Latin “obtrudere”=to thrust against < “ob-“=against + “trudere”=to thrust.

odium: Hatred and disgust. From Latin “odium”=hatred or ill-will < “odisse”=to hate.

off-beat: Unusual, weird, strange. From Old English “of”=away from + “beatan”=hit/thrash. From the musical sense of odd tempo.

officious: Intrusive in a meddling or offensive manner. Latin “officiosus”=dutiful < “officium”=task, duty + “-osus”=full of.

ombrifuge: Anything providing shelter from the rain. From Greek “ombros”=shower of rain + Latin “-fugere”=to flee from.

on: Related to a local position outside of, but in contact with or close to, a surface. From Old English “on” < Greek “ana”=on, upon, or up.

onerous: Not easily borne; wearing. From Middle French “onéreux ” < Latin “onerosus” < “onus+=load, burden + “-ous”=full of.

onomastics: The study of proper names (people, places etc.) From Greek “onomastikos”=pertaining to words.

onomatopoeia: A word made from a sound e.g. clunk, meow, splash, whoosh. From Greek “onoma”=name/word + “poiein”=to make or compose.

onslaught: Sudden and severe onset of trouble. Early Modern Dutch “aanslag”=attack < “aan”=on + “schlact”=slay/slaughter.

onus: Hard or difficult concern; responsibility, burden or duty. From Latin “onus” < burden. Possibly from Sanskrit “anas”=wagon

opposite: Different from something else; contrary to. From Latin “opponere”=set against < “ob-“+against + “ponere”=to place.

opulent: Exhibiting great wealth and richness. From Latin “opulentus”=wealthy.

ornamental: Decorative or created for appearance and beauty. From Latin “ornamentum”=embellishment < “ornare”=to adorn or equip.

ostentatious: Intended to attract notice and impress others; showy. From Latin “ostentate”=to show off + “ious”=full of.

otiose: Unnecessary or futile; being lazy. Latin “otiosus”=ineffectual, inactive, idle < “otium”=leisure, freedom from business.

overwrought: Deeply agitated especially with emotion. From Old English “over”=in excess + “wyrcan”=to work (“wrought”=old form).