It may be worth your while to identify the termites and map the extent of their activity before reaching for a poison. There’s a good chance that something else has happened to weaken the oak’s defenses and let them in, in which case the tree will likely continue to decline even after the termites are gone.
If the termites are active in living tissue, then imidacloprid (Confidor, Premise) as a systemic chemical can wipe them out. It may also be the best choice if they are attacking from the roots (rod it into the ground under the tree, especially at the drip line). It can also be used to flood the large cavities found in infested heartwood.
Your best action though, is to have a professional assess it.