During the 12-hour network event on November 8, Australian individuals and businesses were unable to make calls, access the internet or complete transactions.
In response, the company apologised and customers - including businesses that lost thousands in sales - were offered 200GB of extra data, or free data on weekends if they were on prepaid plans as a "gesture of thanks for their ongoing support and patience".
Ms Bayer Rosmarin opened her address to a Senate inquiry on Friday by acknowledging the pain of her customers.
"While I believe wholeheartedly that we did everything we could to provide timely, accurate and credible information (but) I acknowledge that there is always more we could have done," she said.
"Beyond restoring our network, our focus now is restoring trust."
But the wrath of senators quickly began to fall on the Optus boss.
"You provide a service to over 10 million people and not just individuals - government agencies, emergency services, businesses - and all they got for hours, was a couple of lines that said, 'sorry our services out we're working on it," committee chair Sarah Hanson-Young said.
"That just is not good enough.
"For a communications company, the communication is pretty lousy both in the time of the crisis and in the aftermath."
Greens Senator Sarah Hanson-Young says the Optus response was not good enough.
The Greens pushed for and secured the inquiry the day after the outage and Senator Hanson-Young vowed to examine Optus' responsibility to look beyond their profits and protect the public.
The Australian Communications and Media Authority has also begun its investigation of Optus' compliance with the rules on emergency calls.
Ms Bayer Rosmarin has said Optus will co-operate with reviews launched by the government and the Senate.
The company initially blamed the outage on a routine software upgrade but their updated submission to the inquiry reveals it was the result of its routers' default settings.
The Cisco routers had automatically self-isolated to protect themselves from an unexpected overload of IP routing information after a software upgrade.
"Although the software upgrade resulted in the change in routing information, it was not the cause of the incident," the submission states.
It took longer than expected to restore the system because some of the routers needed to be physically rebooted in a "brute force resuscitation of the network" which required Optus staff to be deployed to a number of sites across the country.
The outage began at 4.05am and was addressed in a crisis meeting at 7.45am, after which Ms Bayer Rosmarin directly contacted Communications Minister Michelle Rowland.
Throughout this period, Optus's 24-hour staff were attempting to amend the issue as the board worried over whether it was caused by a cyber attack.
"There were some strange coincidences that made us quite worried about that," Ms Bayer Rosmarin said.
When Optus was last targeted by a cyber attack, representatives from the telco's parent company Singtel was in town and happened to have been in Australia when this event occurred.
But by 10.20am, Ms Bayer Rosmarin was made aware it was not caused by a cyber attack.
The outage came just over a year after Optus fell victim to data breach that compromised the information of millions of Australians and caused the Medicare, licence and passport numbers of 10,000 customers to be stolen and leaked online.
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