Bark fell off exposing a hollow spot inside. Directly on the opposite side there is a hollow spot as well but the bark is still on. This is a 20 year old live oak.

weird. Seen that before when hit by lightning. May just be disease. That old of a tree may require an arborist. Others may have an idea.

We just had a VERY similar situation at our house in Houston. Tree (oak) looked fine until one day we saw a chunk of bark fall off just like this. We hired an arborist to come out and he said it was wood boring beetles and that it could not be saved. Sucks. Apparently, it's an issue in Houston right now. Had to cut it down and was only tree in our backyard

Hope it can be saved. If so, uncover the root flare.
https://www.dirtdoctor.com/garden/Root-Flare-Newsletter_vq2996.htm

Tractor supply has a root treatment concentre to prevent pests like beetles. I mix up a batch in a 5gal bucket each year and pour on in early spring. Tree absorbs it into its bark and leaves.

https://www.tractorsupply.com/tsc/product/tree-shrub-drench-for-insect-control-concentrate-1-gal-611

That bark looks very ashy for an oak.

How's the canopy looking?

Hard to say it looks sick from just the trunk but I'd get someone to swing by as recommended above.

https://neilsperry.com/2022/03/assessing-live-oak-damage-one-year-later/

That article is reasurring. It is a very small strip, likely less than 5% of the truck circumference. It does not appear that there is any other loose bark anywhere on the trunk, or hollow spots which this one had before it came off.

It's not the Bark I'd be worried about, but the wood underneath looked pretty bad, no?

Actually it looks good. I don't see any signs of boring insects or rot. It looks like the wood inside is starting to form bark.