Mr O'Malley told shareholders at the bank's annual general meeting on Wednesday that CBA is looking at various scenarios for meeting climate targets in a science-driven way, guided by national science agency CSIRO.
"The independent experts have been very clear about the fact that it will be incredibly difficult for the world to get to 1.5," Mr O'Malley said, referring to the Paris Agreement goal of avoiding a global catastrophe by limiting global warming to under 1.5C.
"We must continue on the journey - and it's not going be easy. And we've been very clear about the fact that it's not going to be easy."
Mr O'Malley said that for "large customers that choose not to have a transition plan (on climate change) by 2025, we will not be able to bank them", without giving details.
About two dozen climate activists rallied in the rain outside the meeting at the Melbourne Cricket Ground, the bank's first in-person AGM in three years. A handful briefly disrupted the meeting by singing.
Shareholders activists also questioned, among other things, CBA's recent lending to Santos to finance the $4.7 billion Barossa gas project off the Northern Territory coast.
Mr O'Malley replied that he couldn't discuss individual customers but both he and chief executive Matt Comyn emphasised that the bank, which released its first climate report in August, is taking its climate commitments very seriously.
"We're going to be very transparent," Mr Comyn said. "And next year, we'll begin to see the progress that we've made over the coming 12 months, and our progress and continued progress against that one-and-a-half degree scenario."
While climate concerns dominated the meeting, the rapidly rising interest rates also came up, which Mr O'Malley also described as "incredibly difficult" and a situation that hadn't been seen in Australia in many years.
"We are sort of beyond full employment, but absolutely recognise the concern and anxiety for many customers with rising cost of living with interest rates" and energy prices, Mr Comyn said.
"We're very confident about the overall outlook for the Australian economy but recognise that every individual customer's circumstances are different and we need to take that into account."
Two resolutions, backed by climate activists and not supported by the CBA board, failed as expected. Just 3.2 per cent of shares were cast in favour of the first measure, to amend CBA's constitution, with 96.5 per cent voting against.
Its failure meant the second resolution, on climate risk, wasn't put to the meeting.
