observance, observation Observance means “obedience to a rule or custom”: the college’s observance of Thanksgiving Day. Observation means either “a study of something” or “a remark based on such a study”: his observation about the financial strategy.

obviously Generally unnecessary and best avoided.

occur, occurred, occurring, occurrence

ocean Lowercase when standing alone or in plural uses: the ocean, the Atlantic and Pacific oceans.

off Never put of after this word: we got off the bus.

off-, -off Follow Merriam-Webster’s for hyphenation when using as a prefix or suffix. Some common unhyphenated compounds: cutoff, liftoff, offhand, offline, offset, offshore, offstage, playoff, standoff, takeoff. Some common hyphenated compounds: off-color, off-site, off-peak, off-white, send-off.

off-campus, off campus Hyphenate as an adjective before a noun: She has an off-campus job. Otherwise, leave as two words: He works off campus.

office Capitalize when it is part of a formal name: Office of Environmental Health and Safety. Lowercase in all other uses, including such phrases as the health and safety office, the U.S. attorney’s office.

offline No hyphen.

off of The of is unnecessary: The child fell off the bicycle, not The child fell off of the bicycle.

OK For informal use only; do not use okay (AP style). Formations are OK, OK’d, OK’ing, OKs.

on-campus, on campus Hyphenate as an adjective before a noun: She has an on-campus job. Otherwise, leave as two words: He works on campus.

online No hyphen.

on, upon Use the word on unless introducing an event or condition: put that on the shelf; upon the job’s completion, they will be paid. Do not use the word on before a date or day of the week when its absence would not lead to confusion, except at the beginning of a sentence: The meeting will be held Monday. On Sept. 4, fall classes begin.

Use on to avoid an awkward juxtaposition of a date and proper name: They met Jane on Monday. Also use on to avoid any suggestion that the date is the object of a transitive verb: The House killed on Tuesday a bid to raise taxes. (Such a suggestion can also be avoided by rewording: On Tuesday, the House killed a bid to raise taxes.)

onto, on to, on The distinction between on as a preposition and as an adverb is subtle. The word onto implies a movement, so it has an adverbial flavor even though it is a preposition: the gymnast jumped onto the bars. When the word on is part of the verbal phrase, it is an adverb and the word to is the preposition: the gymnast held on to the bars. One trick is to say up before on: if the sentence still makes sense, then onto is probably the right choice. Alone, the word on does not imply motion: the gymnast is good on the parallel bars.

oral, verbal Use oral to refer to spoken words: Oral tradition is important in all societies. He gave an oral promise. Students describe their research during 15-minute oral presentations. Use verbal to compare words with some other form of communication: Both verbal and nonverbal communication skills are important in the workplace.

outbreak For disease references, reserve for larger numbers of an illness, not a few cases.

outside In spatial references, no of is necessary—or desirable—after this word unless it is used as a noun: outside the classroom; the outside of the building.

over- In general, no hyphen when used as a prefix: overrated, override, overshoes. See Words Formed with Prefixes under General Style Preferences.

over, more than Over generally refers to spatial relationships: The plane flew over the city. Although the use of over as an equivalent of more than is perfectly good idiomatic English (Chicago Manual) and is now acceptable in AP style when indicating greater numerical value, more than is preferred: The Purchase College Library offers full-text access to more than 60,000 electronic journals, magazines, and newspapers. More than 500 people attended the concert. In general, use more than when the opposite is less than or fewer than.

Updated June 27, 2014