When you save your banking PIN for the individual bank credentials, it will only be saved locally on your device - encrypted. This enables your app, among other things, to refresh data in the background and to notify you on new transactions. 

As all data is only saved on your device, you are the only one who can access it with your App password - no one else. Not even we as the developers. If you wish to use Outbank on multiple devices with the same data, you can activate 'Secure Sync' in the app settings. In this case, all your data is additionally stored on Amazon S3 cloud drive - even there, everything is encrypted at all times. Find further details here.