Just finnished the configuration of my test Office 365 environment.
I was interested in testing Lync Online and Federation with Lync On-Premise
To do this i had to enable federation in my Office 365 Lync Online Control panel,
and once this had been done i needed to go to my On-Premise install of Lync and complete the following command:
New-CSHostingProvider –identity LyncOnline –ProxyFqdn sipfed.online.lync.com –Enabled $True
This enables the Lync On-Premise to federate with Office 365 Lync Online