The phrasal adjective one-time means occurring only once, and onetime is synonymous with former.
One caveat: The onetime–one-time distinction does not necessarily apply outside the U.S and Canada. In British publications, it seems the hyphenated one-time is commonly used to mean both occurring only once and former (with exceptions—see the third example below). So writers outside North America can safely avoid onetime entirely. In U.S. and Canadian writing it is clear onetime and one-time are generally treated differently, even if some dictionaries have not caught up to the difference.
Examples
These writers use onetime well:
The onetime Texas closer—he lost that job last year to Neftali Feliz—has worked just two innings this spring … [National Post]
But it may not be enough to save the onetime king of social networking, which has been dethroned by Facebook and faces competition from other sites, too. [USA Today]
The book as a whole explores the lives of a decrepit punk rocker and his onetime PA. [Guardian]
And these writers use one-time in its modern sense:
The measure … requires the government to levy a one-time tariff on bonuses paid since 2008 … [Financial Times]
What we didn’t assume was that the agreement would be shot through with gimmicks and one-time savings. [National Review]
There is a one-time enrolment fee of $300, and a $25 per month service fee to cover new batteries and repairs … [Globe and Mail]
