Bank staff don't know anything about immigration. OX visa is simply not used.
In order to open a bank account they just need to be on a long-term visa or extension. Even a non-O visa suffices, and it's only 90 days. Those 90 days are meant for opening the bank account you must have in order to get the 1-year extension.
But each bank branch manager sets their requirements for that bank branch. There are many that simply won't deal with foreigners so they will make rules that are impossible. Just go to the next bank branch down the street and try there. Eventually you'll find one that will open a bank account with a long-term visa and a residence certificate from immigration.
Don't even bother trying if he's in Thailand as a tourist, as banks no longer open accounts for tourists.