OBE (and other honours)

no need to use in publications unless context demands – for example, a news story about New Year Honours. But note that people can be sensitive about their honours. If in doubt about their preferred form of address, check with them.

occur, occurred

Office for National Statistics

ONS at second mention

OK

not ok or okay

older

avoid “old people”, “elderly people” and “the elderly”

Olympic Games

or Olympics

on call

two words

one in six, one in 10

should be treated as plural  eg “one in six patients are obese”. “One in six” represents a number of people, a group rather than an individual

ongoing

no hyphen, one word

online

on to

not onto

outpatient, inpatient

From The Guardian style guide: “St Thomas' hospital in south London boasts the following styles, all on signs within a few yards of each other: Out Patients, Out-Patients, Outpatients, and outpatients.”

outside

not “outside of”

overestimate

take care that you don’t mean underestimate

overstretched

Oxford comma

comma before a final “and” in lists – only use if it can help clarify meaning. Compare “I dedicate this style guide to my parents, Peter Carter and Sandra James” with “I dedicate this style guide to my parents, Peter Carter, and Sandra James”