With thanks to Chris for spotting this, there is a problem using the OSI between platforms 2-4 and platform 1 at the north end of Blackfriars NR station. If the journey being made is entirely on National Rail the system will instead charge as if you’ve used the Underground as well. Fortunately there is a way around this because you can switch between all platforms at the south end of the station without needing to go through a barrier.
The problem is that the indicator that you’ve changed between NR and LU at London terminals is a touch on either the NR or LU gatelines. Originally this was not a problem, but since the rebuild of Blackfriars station and the need to exit and re-enter the paid area at the north end it is now possible to need to touch in/out without actually using the tube.
As an example, a journey between Elephant & Castle and Greenwich using National Rail via Blackfriars will be charged £3.10/£2.60 if you use the southbank subway at Blackfriars or £4.70/£4.10 if you use the northbank.