With my own views,
I haven't heard Oracle alone is being used to implement such wide range of scope of that kind of project. Some small banks uses Oracle for Banking, but never heard Oracle being used alone with affiliation world wide.
I know BASE24 was widely used during late 1990s together with IBMs DB2/AS400,etc.
Its got alot to do with the whole infrastructure not just the software but also the hardware aspect.
Kind of transactions such as ON-US transaction- your banking transaction is with the same bank in which your account belong. And the other one is your banking transaction is made from a bank affiliated with organization of different banks where your bank is a member, such as BANCNET,etc. Where in your initial transaction info such as balance is immediately forwarded by BANCNET to your bank and later-on there will be a CLEARING in a certain time.

I hope this helps you somehow.