Chief executive Stephen Rue was in the firing line from coalition and crossbench senators for taking more than six hours to tell the communications minister and industry regulator about a massive increase in the scale of the September outage.
It prevented more than 600 triple-zero calls from connecting when Optus originally suggested the number involved was just a handful.
The telco announced on Monday that 300 people would be added to its Australian call centres with a focus on the emergency network, while safeguards surrounding triple-zero calls would be ramped up following the incident.
Liberal senator Sarah Henderson said the deaths were preventable after Mr Rue said the company regretted not moving sooner on reforms that would have detected the outage earlier and better protected customers.
"Optus never detected the outage, which I'm shocked about ... for hours and hours and hours, Optus did not know what was going on," she said.
Greens senator Sarah Hanson-Young questioned why senior Optus management were not told for hours that multiple deaths had been linked to the outage.
She also queried why there was a six-hour delay in notifying the Australian Communications and Media Authority or Communications Minister Anika Wells when Mr Rue was personally informed about the seriousness of the issues.
"What on earth were you doing between 8am and 2pm?" Senator Hanson-Young said.
"You were too busy putting your ducks in order, telling your board what was going on, contacting your executives ... meanwhile, the federal government, the regulator and the minister, were left in the dark."
Mr Rue defended the delay, arguing Optus was conducting welfare checks and wanted to provide government officials with accurate data.
"The judgement I made was it was best to get the information accurately together and then inform the regulator, the department and the minister's office."
Mr Rue, who has faced calls for his sacking after the outage, said the introduction of new executives could hamper the work Optus had done to increase triple-zero network protections.
Company chair John Arthur backed the chief executive to keep his job.
The triple-zero outage was caused by human error during a routine firewall upgrade, meaning triple-zero calls were not diverted to another network, officials said.
Representatives from the communications watchdog will face the inquiry later.
The probe was set up to better understand what caused the September outage, which stopped hundreds of Australians from making triple-zero calls.
It will also examine the effectiveness of emergency arrangements designed to shift customers to another network if their telco has an outage.
The communications watchdog and Optus are both running their own investigations into the outage.
Rules that took effect on Saturday require telcos to report outages to the communications watchdog and emergency services in real time.
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