There is no native integration. But as of Update 2, you can trigger a release from the command line. Which in turn will allow you to modify your build process (assuming your build agent is on-premise and not hosted) to trigger the release automatically. The following blog post provides details: http://blogs.msdn.com/b/visualstudioalm/archive/2014/04/04/release-management-continuous-deployment-with-visual-studio-online-and-any-ci-systems.aspx
Update:
Microsoft is working on bringing the Release Management client features into the VSO Web inteface. This was announce in February and named as Release Management Service of this year and a working demo was show on \build of this year.
Source:
Latest Blog Announcement: http://blogs.msdn.com/b/visualstudioalm/archive/2015/04/29/release-management-announcements-at-build-2015.aspx
In depth demo of UI and new features (relevant part starts around minute 25): http://channel9.msdn.com/Events/Build/2015/3-649