I don't have any experience with OutScale OSU but as long as it has good compatibility with the S3 REST API, and as long as you don't use any features specific to Amazon's S3, then JetS3t should work with it.
Having said that, over the years a number of small issues have come up where services that claimed compatibility with S3's API didn't work properly with JetS3t either because the service did something unexpected, or because JetS3t itself was too fussy expecting things to work exactly like S3 when it was reasonable to do them slightly differently. For example, handling of HTTP headers is one area where JetS3t has been too fussy in the past.
Hopefully all those kinds of issue have been fixed by now, but you can never tell for sure with a new service until you try it.
It's great that you have a proof-of-concept working. I would recommend that in your PoC you test both the main S3 features you rely on against OutScale OSU, as well as any minor features that are more complex and therefore might have issues, just to try and flush out any issues as early as possible.
Good luck with the migration, and please follow up here to let us know how it goes.