m. Living near O, I would say that does present a few issues. How early can you get there? I think I would probably try Seacourt P&R. It does mean having to go through the A420 Swindon/Botley roundabout which is currently single lane only and can be v busy. Then the P&R bus will stop at Osney and you'll need to walk in from there, under the railway bridge. You could take another bus from the station if the walk from there is too long.
If you do that then the half marathon route itself won't affect you. But the earlier you can get to O, the better.