Once and One-time

Once adjective - Having been such at some previous time.

One-time is a synonym for once in bygone topic. In some cases you can use "One-time" instead an adjective "Once".

One-time adjective - Belonging to some prior time.

Once is a synonym for one-time in time topic. You can use "Once" instead the phrase "One-time" as an adjective or an adverb, if it concerns topics such as bygone, former.