Sorry, I should have added some figure legends to help explain, and probably also posted the pictures In a logical order! So here are some notes:
Photos 1 and 3 are looking in the down direction, taken from what is now platform 2, roughly where the buffer stop used to be on what was former platform 3. The old platform canopy which dated from GW days has been removed completely. Where the new platform 1 is (opposite), there used to be a couple of goods bays. A CCT and GUV were kept there for years, used as stores. The new building behind the new platform 1 is, I think, offices and the crew depot.
In photo 3 you can just make out a Chiltern 168 in the siding undertaking driving training. The bridge is not technically Castle Mill Stream, I think, but the 'cut' between the Stream and the Thames. The former Oxford depot is to the left, where the Portakabin buildings are - all of the area now forms the down stabling sidings.
Photos 2 and 4 are looking back in the up direction towards the main station building. At the right in photo 2 is the footbridge across from what is now platform 3 (used to be 1!) to what is now platform 4 (used to be 2!). The last dozen yards or so of the new bay platforms 1 and 2 used to be the short stay car park, so the erstwhile bay platform here has indeed been lengthened fairly substantially. There used to be a bay on the down side too, which was also used for the Fairford/Kidlington branch services.
Hope that all makes sense - let me know if anything else needs clarifying. I last visited the station about six weeks ago, and I was stunned by how much has happened in that time!