These are sawfly larvae; possibly the Scarlet Oak Sawfly (Caliroa quercuscoccineae). Despite their name they are not limited to Scarlet Oaks and can feed on a number of Oak species. They are not harmful to the overall health of the tree and do not require control. If this tree is young enough that the leaves are within reach, simply pick off those with larvae and dispose of them. Pesticides (including organic sprays) may work but are rarely warranted; they too require you to be able to reach the pests in order to spray them. Oaks are host to a number of insect larvae which are of invaluable importance to birds and other predators (such as beneficial insects), which is why insecticide use is not desirable. Given that, a number of these larvae will probably disappear before reaching maturity.
https://hortnews.extension.iastate.edu/oak-sawfly
https://www.marylandbiodiversity.com/view/9701

Miri