Oxford crews also work this line, but I don't think they know the route beyond Malvern, so not on Hereford trains.
Half of the drivers that know the route to Malvern also know the route to Hereford. Though in the case of the two principal evening services from Paddington-Hereford they are not rostered for Oxford drivers - though when there are shortages or delays Oxford drivers may get asked to cover them.
The same applies with the Train Manager when Worcester staff may be 'stepped up' to cover.