Most probably a sap sucking insect like an aphid or similar. They squirt a concentrated sugary solution out of their rears which lands on all below. Oaks can fare reasonable well with the onslaught unless they have other problems at the same time such as drought!

It should be ok. Oaks (Quercus robur and Q. petraea) our native two species harbour more species of insect than any other native tree and are a boon for many types of wildlife both directly and indirectly. Hopefully, the local Titmice will be feeding up chicks on these sap suckers now.

We had this one very hot dry summer on our oak trees - just dont park your car underneath as we did.