OCBC apologised to affected customers for the outage that lasted about three to four hours last Saturday night.
The bank said in a media statement that the incident was due to a software failure that was not detected "due to a human oversight". It affected the bank’s ATM network, online banking channels and card services, as well as electronic payment system Nets.
Disruptions occurred intermittently from 8.45pm and worsened at 8.53pm when all banking channels went down.
Over 1,000 customers contacted the bank’s helpline during the outage. OCBC expressed its apologies on its social media channels for the inconvenience experienced by customers.
The bank added that investigations found that a software failure in the backup unit for the core banking system had resulted in customer transaction data not being transferred successfully. As a result, the storage in the core banking system reached its maximum capacity and stopped accepting new transaction data.
This was not being detected, causing the core banking system to be unable to function properly, and in turn affected its various services.
OCBC's chief executive officer Mr Samuel Tsien noted that the disruption was "totally unacceptable", adding that the bank had already put in place measures to prevent any future recurrence.
“I am grateful for the patience and understanding of our customers and I would like to sincerely apologise to all our customers again for the inconvenience that this incident has caused them,” Mr Tsien said.