Check your bill for that info.
But content services according to Optus are 3rd parties who provide a service to you and bill your mobile bill. Think about those late night ads you see on TV where you can have your fortune told if you SMS your name to 19xxxx. These days it's mostly ads you'll see on websites, FB etc that can sign you up quite easily.
Either way, if you want it to stop and to dispute it, go to live chat - http://yesopt.us/chat2us
First thing you do is tell them to set your spend limit to $0 so this can't happen again. Second thing is to advise them you didn't ask for or use these services and you want to dispute them. They'll lodge the dispute with the provider in question who then need to contact you within 2 business days to discuss and work out a resolution which is normally a full refund.
Actually appologies, my above response might be correct however do you have a Fetch Mini by any chance? That's $5 a month ($4.55 pre gst) and shows up under content services. I thought it was weird to but I found out on my first bill
As SamSam is completely correct with this one. On your invoice there is a description of the charge. Bills can be viewed online via your Optus My Account. If you’re having trouble locating your bills, further instructions can be found here. If you would like us to check for you, please send us a private message confirming your phone number, full name and date of birth. Thank you 🙂