Edit: minor change
Doesn't look like A. kollari as it's growing on the leaf. Looks more like a currant gall caused by Neuroterus quercusbaccarum. If so, there will be a smaller convex bit of the gall on the topside of the leaf.
It's probably still too early for cherry galls (Cynips quercusfolii). But these would be on the one side of the leaf only.
Thanks. Please feel free to add the more likely name.
University of Edinburgh and Biodiversity Observatory (OU)
Don't suppose you have another photo of this showing top side of leaf? Pretty certain it is this though.
These galls often appear on the male catkins, as well as the leaves, where they look more currant-like.
There's a small oak in our garden with quite a few of these on (photo of which I put on iSpot a week or two ago.)
Same species causes common spangle galls on underside of leaves in autumn.
Sorry, Dave, but no. This was a picture taken on the fly.
Lat/Lng: 52.7129, -2.2395444444444
OS grid ref: SJ839128