I do apologise for any inconvenience caused. There should be no issues with using Chromecast while watching Optus Sport. I've attached some more information here →https://goo.gl/JNRZEt in regard to how this works. You'll just need to click on the 'How to watch on your TV at home' tab.
The casting symble won't appear if Chromecast is not turned on and connected to the same wifi as you phone or iPad.
Might be a simple solution but easy to do, for me anyway.