If you're in no hurry then get to the west of Witney, there's a very nice route along the Windrush valley towards Burford, via Minster Lovell, Asthall and there are a couple of relatively easy crossings to be had. Alternatively on the eastern edge of Witney the road to South Leigh goes under the racetrack. Otherwise as bikepacker says there's a light controlled crossing at the Cassington junction.