Moved from "Parasitica" (parasitic Apocrita).

This appears to be an immature larva of Arogalea cristifasciella as you suggest, based on Robert Marquis' description of Lepidopteran larvae of oaks written in 2019. The current literature suggests that an ichneumon wasp in the subfamily Lycorininae parasitizes this particular host. That does not exclude, of course, the possibility of other wasps targeting this host. Due to the scarcity of images of most parasitoid wasp larvae, it might be helpful in the future, when you encounter these larvae, to collect them for rearing, so we might catch an adult emerging, which would be much easier to identify.

See host reference here.

Moved from Ants, Bees, Wasps and Sawflies.