You may want to get the opinion of another certified arborist or licensed tree expert who can examine the tree. Part of the prognosis may depend on the species of tree (which species of oak it is, for instance), since they can differ in how well they deal with decay or damage. Arborists have more training than we do when it comes to matters of tree health, and individuals with a TRAQ certification (tree risk assessment qualification) may be better-suited to determine how at-risk a tree with wood decay is for falling.

The void is showing signs of wood decay, but by itself, that's not an urgent situation. For one, it likely began many years ago, likely due to whatever injury first damaged the bark on either side of the tree. Trees have ways of walling-off areas of decaying wood to prevent it from affecting live wood, although this process (compartmentalization) isn't visible from outside the tree.

It's not recommended to seal, paint, coat, or otherwise cover-up any wound in a tree, nor is it recommended to fill voids with anything, which can trap moisture and decay-causing organisms inside. If several certified arborists make the case for using foam or some other gap-filler, we defer to their expertise, but generally it's not a good practice, nor does it help the tree avoid any progressive damage if the decay is still spreading. Carpenter ants don't generally excavate intact, live wood; they merely tunnel into damaged and decaying wood that is already deteriorating from moisture, fungi, or other factors.

Wild trees in natural areas routinely keep growing for decades with part or most of their trunk having hollowed-out after years of wood decay. The hollow areas might make a tree more vulnerable to storm damage, but won't necessarily impact it's ability to grow and continue to produce leaves each year. The innermost wood in a tree's trunk and large branches is naturally already dead tissue; the only living layer is a few inches of wood just under the bark, which transports sap carrying moisture and nutrients back and forth between the canopy and the roots.

There is usually little a non-arborist can do to improve tree health, in part because most problems are not treatable or curable by the time they manifest obvious symptoms. Trees cannot regrow wood in the inner trunk (the tissues only produce new wood going outward from the current size of the trunk), so they cannot replace any wood lost to decay. Organisms like certain insects and fungi might continue to decay wood in a cavity without posing a direct risk to tree health, and tree voids can be valuable for wildlife as well. (Perhaps less so when they are this low on the trunk, but many animal species take shelter or nest in such places that protect them from predators and the weather.)

While helping to alleviate any drought stress by watering periodically (our ongoing drought is far from over yet) may help the tree, the root area is so expansive on a tree this size that it's usually not practical to try irrigating it. The only other proactive protective measure you can take is to make sure any mowing or string trimmer equipment used in that area, since there is no mulch surrounding the trunk's base, does not injure the bark, which could create wounds that get infected with a pathogen that does kill live wood (or wounds that get colonized by insects that actively tunnel into live wood), neither of which could be treated once they colonize the wood.

We don't see a root flare on this tree, which suggests it was buried too deeply at some point (when it was planted, if it's not growing wild, or perhaps during home construction if the tree was already there when the area was developed). Girdling roots might be present, which can cause decline and canopy dieback. Some girdling roots can be cut to reduce the "strangling" effect they have on the trunk or on other roots, but when girdling roots get too big or embed themselves in the trunk base, there's not much that can be done to remedy the situation. Arborists can help locate the root flare and look for problem roots, but sometimes the equipment needed (and air spade) and the time it takes to excavate the soil closest to the trunk adds up to an expensive procedure for larger/older trees. You can learn more about the importance of root flare health for trees on the three pages linked above.

Miri