As mentioned by @Tigers1719, the default will be 0000.
If you're still having troubles with this, our Customer Care team can reset the PIN for you.
Please note that we’re unable to look into account details on Yes Crowd. We recommend messaging us via the My Optus App. The team is available 24/7.
If for some reason you’re unable to, if you have a social media account, please then send a PM to Optus on Facebook or a DM on Twitter with the details for further assistance mentioning you’re unable to use the My Optus app.