O

offline, online
One word in all cases for the computer connection term. Also, see Computer terminology.

OK, OK’d, OK’ing, OKs
Do not use okay.

Olympics
Capitalize all references to the international athletic contests: the Olympics, the Winter Olympics, the Olympic Games, an Olympic-sized pool, but lowercase the games when used alone. Lowercase other uses: If there was knitting olympics, her cable stitch would win a gold medal

one-
Hyphenate when used in writing fractions: one-half, one-third. Use phrases such as a half or a third if precision is not intended. 

Orient, Oriental
Capitalize when referring to the Far East nations of Asia and nearby islands. Asian is the preferred term for an inhabitant of these regions. Also, see Hyphenation, dual heritage.

over
See more than.

overall
A single word in adjectival and adverbial use: Overall, the Democrats succeeded. Overall policy.