Since Exchange 2007 it’s possible to use your OCS and Exchange together. First you had only the option to use your OCS environment with the Exchange UM server which let you use the voicemail, subscriber access and auto attendant features from Exchange in OCS
The latest addition to this is a limited OCS client integrated in OWA. This gives you the ability to send IM messages via OWA and gives you the option to see precense information.
In this tutorial I will explain how you can install this new feature.