When you click the 'usage tab,' is the pie graph displayed? What happens when you navigate to the 'usage breakdown' screen?
Are you prompted with an error message at all?
It might be worth clearing the app cache:
Go into Settings > More > Application Manager
Select the My Optus app
Select Clear Data and Clear Cache
Uninstall and reinstall the My Optus app using Mobile DATA only
Open iPhone notes
Type myoptus://clear and then click Done
Click on the link that appears in iPhone notes
Uninstall the My Optus app & restart the phone
Reinstall My Optus app using Mobile DATA only
Failing that, we'll need to take a look at the account from our end. You're welcome to send us a PM with your account details. We're after your full name, DOB and mobile number.