To get to South Northamptonshire by car from Oxford, take the A43 and follow the signs to Northampton. Continue on the A43 and follow signs to Luffield Abbey and Towcester. From there, take the A5 towards Weedon and follow signs to your destination.
To get to South Northamptonshire by train from London, take the West Midlands Trains service from London Euston to Northampton. From there, take a taxi or bus to your destination.
If you are coming from nearby towns such as Banbury or Milton Keynes, driving is likely the easiest option. Alternatively, there are buses available from Banbury and Milton Keynes to Northampton, from which you can take a taxi or bus to South Northamptonshire.