I’ve just been doing some testing between our Lync On-Premise installation and Office365 Lync Online with Thomas Lee (who is all over Office365 information)
The information about implies that Lync Online, once you choose Open Federation on your web admin console, will federate with any On-Prem Lync installation that also has open federation. We found this not to be the case. Our system is Open federation and already federated to Microsoft (not that that appears relevant), but initially adding contacts didn’t give presence.
Initially no presence
The solution is that you need to add a new Hosting Provider for Lync Online.
By default there are 3 Providers
To add Lync Online either click the new hosting provider,
Or to be quicker launch the Lync Management shell and run the command:
New-CSHostingProvider –identity LyncOnline –ProxyFqdn sipfed.online.lync.com –Enabled $True
Once you refresh the Lync Control Panel you should see your new hosting provider
Once the CMS has had a minute or two to sync, everything worked fine
Credit where credit is due, I found the solution here
So it seems On-Prem deployments will need to add this hosting provider to allow anyone using Office365 to federate.