In Release 12 Bank Accounts are owned by Legal Entities and can be accessed by multiple Operating
Units. In Prior Releases the Bank Accounts were Operating Unit Specific.

For all Internal Banks you must assign a Legal Entity.
If the Legal Entity has been configured and it does not appear in the List Of Values in the Bank Account setup then you may need to run the CE UMX Security Wizard to ensure the Legal Entity is available to the Responsibility you are using to access Cash Management.

CE Bank Account Security setup wizard
1) Login as System Administrator
2) Select User Management responsibility
3) Select Roles and Role Inheritance menu function
4) Create (or query an existing) Role.
5) Click Security Wizards button and Run Wizard - CE UMX Security wizard
6) Click Add Legal Entities button, select your LEs and check Use, Maintenance and Bank Account
7)Transfers to reflect the activities for this Role.
check-box. Apply
8) Assign this role to your user (using the Users tab)

Banks in R12 is defined in legal entity level. Each branch has many branches and account details in a bank branch specify all the oracle relevant data.