In 21st-century North America usage, one-time, with a hyphen, means occurring only once. For instance, a payment you have to make only once is a one-time payment. Onetime, without a hyphen, is synonymous with former. For example, George W. Bush is the onetime president of the United States. You will not see the one-time/onetime distinction noted in dictionaries because it is a new development, not widely borne out until this century.
Outside North America, the hyphenated one-time is commonly used to mean both occurring only once and former (with exceptions—see the third example below). Onetime does appear rarely, but it doesn’t have a meaning all its own.
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]
One-time (occurring only once)
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]