Your phone will still report data usage (cause you are using data). The Optus app will also show data usage in the app summary screen (even for those that are part of the free streaming).
The key word is not data but free - Optus don't count or charge the free data. The best way to confirm is the overall data usage screen in the Optus App (that just tells you how much your plan has and how much you have left). This won't go down much if you're using free streaming.
What are you streaming (SD Netflix?)
Hi @C-pat26 you will need to have your data activated on your phone in order to steam online. Please bear in mind that while usage will appear via your Optus My App, you won't be charged for this data provided it's via a promoted application - Spotify for example.