Ms Bayer Rosmarin tendered her resignation on Monday morning.
"It's been an honour and privilege to lead the team at Optus and to serve our customers," she said in a statement.
"I was also able to communicate Optus' commitment to restore trust and continue to serve customers.
"Having now had time for some personal reflection, I have come to the decision that my resignation is in the best interest of Optus moving forward."
Chief financial officer Michael Venter will assume the role of interim CEO while continuing in his current position.
Former Optus business managing director Peter Kaliaropoulos will become the company's chief operating officer from Wednesday, a newly created position that will report to Mr Venter.
Ms Bayer Rosmarin had attempted to alert Communications Minister Michelle Rowland of her resignation prior to the announcement but was unable to reach her as she was in cabinet.
The two have since spoken about the interim arrangements at Optus and Ms Rowland sent her good wishes.
"This was no doubt a difficult decision and we wish her well for her future endeavours," Ms Rowland said on Monday.
"The Optus outage experienced earlier this month caused significant disruption to the community, particularly small businesses.
"We need to learn the lessons to ensure industry and government is as prepared as possible, given no network is fully immune."
The government has announced it will conduct a review into the outage.
Ms Bayer Rosmarin was appointed to the top job in April 2020, during the height of the pandemic.
In September 2022, she presided over one of the telco's worst PR disasters when it suffered a massive data breach that affected 10 million Australians and resulted in the driver's licence, Medicare and passport numbers of 10,000 customers being stolen and leaked online.
About 14 months later, just as the company had begun to piece together the remains of its customers' shattered confidence, Optus was hit by a 12-hour long outage that prevented more than one-third of Australians from accessing the internet and receiving cell service.
More than 200 emergency calls could not go through to the triple-zero line.
Yuen Kuan Moon, chief of Optus's parent company Singtel, acknowledged the former CEO's leadership, commitment and hard work during this "challenging" period and thanked Ms Bayer Rosmarin for her dedication.
"Kelly has always led with integrity and had all stakeholders' best interests at heart," he said.
"We understand her decision and wish her the very best in her future endeavours."
Following the November 8 outage, Ms Bayer Rosmarin was summoned to a parliamentary inquiry where she was roasted by senators channelling the ire of their constituents.
During Friday's hearing, Nationals senator Ross Cadell queried her response during and after the network event and asked whether it was time for new leadership at the telco.
In response, she said she would "take that on board".
Greens senator Sarah Hanson-Young, who chairs the Senate inquiry into the outage, thanked Ms Bayer Rosmarin for answering questions.
"This was never about which individual is CEO," Senator Hanson-Young said.
"This is about ensuring millions of Australians have access to what is an essential service; including the ability to call 000 in an emergency, access government services, contact loved ones, and make and take essential payments."
The Senate inquiry will continue examining solutions, such as stronger regulations for telecommunications companies, Senator Hanson-Young said.
