When you need a noun referring to (1) the act or process of rolling over, (2) an accident in which a motor vehicle overturns, or (3) the instance of rolling over funds, use the one-word rollover. The word also works as an adjective in phrases like rollover funds and rollover accident. It is never a verb. For all verb senses, use the two-word phrasal verb roll over. For instance, one might roll over his or her rollover funds.
Because rollover is an adjective, there is no use for the hyphenated roll-over. Yet the hyphenated form is often used, especially in British publications, in place of the one-word form.
EU officials may try to persuade investors to roll over maturing debt by offering them preferred status. [Sydney Morning Herald]
A medical episode may be to blame for a rollover crash Thursday morning. [CTV]
But one politician out there would roll over this hapless group like a tractor through a cornfield. [Politico]
The contract is structured with an automatic rollover each year. [USA Today]
The Taliban could roll over the country as the Vietcong did in Vietnam. [Guardian]