I'm concerned about the possibility of an unauthorized person porting my number away from Optus. Is there any way that I can require additional confirmation or security before a port out can be completed?
Unfortunately not. Telcos like Optus and Telstra are hamstrung by the rules set out by the ACMA that were originally designed to ensure a fair and competitive playing field across all telcos but are outdated now. Right now it'd be seen as anticompetitive if Optus put measures in place that increased the amount of time it took for you to port out to another carrier.
The good news is it's a priority for Australian telcos to rectify and I'd hope the same goes for the ACMA as well. I've also heard that the banks are working with telcos to help reduce the chance of fraud as well.
Some more recent discussion about this can be found on this thread.
I believe Optus has recently implemented additional security measures for Prepaid Mobile services.