no difference

what's the different between once and one time ?

The only difference I can think of is that once sounds a bit more formal than one time.

I will do it one time. I will do it once. In this case once is an adverb and they are the same. Once can also be an adjective, conjunction, or noun where sometimes it can be replaced with one time and other times not. See the examples of once here: http://dictionary.reference.com/browse/once?s=t It could be swapped with one time in "Once is enough", but not "Once you're finished,“ or "The once and future king". Once is also used in many idioms shown in the link, which cannot be replaced with one time either.

Probably they have same mining. .its depend on situations