it's actually extremely difficult to do, you need the person to be using a cracked sms app. and usually people use the default app for sms, iphone use imessage, android use the default one provided by os
if anything i'd say the otp is legit, just that hacker managed to bypass or delay the sms trigger on ocbc end, either via ddos or sth
Actually the simplest explanation is
They cloned the ocbc website design, triggered an sms using spoofed phone number that used url shortener that directed to the fake site.
Got a list of ocbc customers or some banking list sold on those dark web. Did the above, then the customers went in, type in login credentials, auto "login". Real otp sent to user's phone. User typed the otp on that fake site and voila.
Not sure if additional step was done to show the balance, but this would have been quite easily done also, only needs the hacker to edit the figures, so long as he or she has the ocbc website interface after login
might need to insert a "delay" tactic here during login so that the OCBC users will not suspect for that 10-15seconds
This move can potentially delay the more "savvy" IT users response and cause them to call the bank only much later
The other end, they spoofed singapore IP maybe? Then the hacker just typed in credentials and otp.
How this didnt trigger ocbc's alarm bells is worrying, how the large amounts of money simply sent overseas just like that.
I think this was a carefully planned one time operation... Done by like those scam centers u see on youtube