Is coworker one word or hyphenated?

In the US, coworker is normally not hyphenated. However in the UK, it is usually co-worker.

British English and U.S. English Hyphen Usage

There are some differences in the manner in which the hyphen is used in the United Kingdom and in the United States, although these differences actually are rather minimal. For example, in the UK certain prefixes will be hyphenated when they will not be so separated in the US. For another example, the Word preschool in the US is hyphenated pre-school in the UK.