offensive In Britain, offensive (as an adjective) means rude; in America, it often means attacking. Similarly, to the British an offence is usually a crime or transgression; to Americans it is often an offensive, or the counterpart to a defence.
oligopoly Limited competition between a small number of producers or sellers. See also monopoly, monopsony.
one Try to avoid one as a personal pronoun. You will often do instead.
only Put only as close as you can to the words it qualifies. Thus These animals mate only in June. To say They only mate in June implies that in June they do nothing else.
onto On and to should be run together when they are closely linked, as in He pranced onto the stage. If, however, the sense of the sentence makes the on closer to the preceding word, or the to closer to the succeeding word, than they are to each other, keep them separate: He pranced on to the next town or He pranced on to wild applause.
overwhelm means submerge utterly, crush, bring to sudden ruin. Majority votes, for example, seldom do any of these things. As for the ethnic Albanians in Kosovo, although 90% of the population, they turned out to be an overwhelmed majority, not an overwhelming one, until NATO stepped in.
oxymoron An oxymoron is not an unintentional contradiction in terms but a figure of speech in which contradictory terms are deliberately combined, as in: bitter-sweet, cruel kindness, friendly fire, jolie laide, open secret, sweet sorrow, etc.