There is a cyclepath that goes round the south side of the roundabout, it is signposted to Steventon.
Heading into Didcot the path follows the righthand side of the road all the way through to the next roundabout.
Didcot railway centre is signposted left at this roundabout but there is no cycle track across the railway bridge there.
Just up the road from the McD roundabout there is an underpass that takes you tunder the railway line and into Milton Park.
Personally I would take this then use park Drive and Milton road to bring you back up to the roundabout you'd come to going the other way. Basil Road after the roundabout is where the Railway centre sign sends you.
Was along that way again today heading for the Tesco Depot