We’ve been providing means to migrate your data from VSO to an in-premise TFS, but not the other way around. Until now!
We just announced a new integration service from OpsHub that allows you to migrate your data from your on-premise Team Foundation Server implementation to Visual Studio online. Full details can be found here, in the Visual Studio Gallery.
While you’re at it, take a look at the other capabilities that OpsHub offers. They have an amazing line-up of ALM systems that they can integrate with their Integration Manager solution.