You could do Oxford with relative ease, trains more or less to and from Marylebone, journey approx 80 mins and Paddington journey approx 55 mins. Trains run every 15 minutes give or take a minute or two. Trains to London run till 23:15 - 23:30. Get a 1 or 5 bus from outside the station which will take you to a stop, Longlands Rd which is about a 5 minute walk to the stadium. Buses run every 5-10 minutes.
NDL meeting on sunday 26th April Chargers vs Middlesborough 3pm start, weather etc permitting, so you could see how easy the journey is with a lot more leeway for getting back to London.