Lync Online recently expanded is reporting capability. To date, there are numerous Lync Online reports such as peer-to-peer session report, conferences report, active users report, and audio / video minutes report - http://technet.microsoft.com/en-us/library/dn362827.aspx. All of these reports are available visually through the admin center plus there are REST web services and PowerShell available to pull the data.
Lync Online has just new report called the Client Devices report - http://blogs.office.com/2014/10/17/announcing-lync-online-client-devices-report/. This report provides data about what types of devices are being used to connect to Lync Online.
All of these reports will provide you good insight into how the Lync Online service is being utilized by your end users.