I would consider consulting an arborist to find out what is on your oaks and whether/how to treat it. Willynilly spraying of pesticides can be a problem for other beneficial insects and other invertebrates ranging from ladybugs to bees to worms. Pesticides can also interfere with some of your soil's flora and fauna that helps keep it healthy. An arborist may have a treatment that can be injected to keep your trees healthy that will have a narrower impact that sprays which spread farther.

Ditto what babs said. You need to first identify the bug you are seeing, determine if it is doing damage to your trees, then choose an appropriate course of treatment that will specifically target the pest in question.

Your town's department of natural resources, or your county extension service, can identify the bugs for you - just slip one into a plastic bag and see who's available to take a look.

I once came within inches of spraying a line of spruce when I found a large number of tiny beetles that I couldn't immediately identify - the spruces were obviously suffering and I didn't know what was doing the damage. One more look on-line, and I discovered that the beetles were immature lady bugs; they were there to save the spruce from spruce spider mite. I've always been careful about using chemicals in my yard, but since that day, pesticides are really my last resort. Your beetles are obviously not lady bugs, but they're not necessarily the culprit.

The site I'm linking to has lots of photos, and you may be able to get an ID, if you have pics of the top side of these guys.

Here is a link that might be useful: North American Insect ID site

Thanks "diggingthedirt" to recommend an insect ID site. But I cannot find a category that fit the beetle that I found.
I attached another photo here.

OK, I don't think that your bug is the problem. It looks to me like the non-lighting, daytime "lightning bug" Ellychnia corrusca. The adults overwinter by crawling between the cracks in tree bark.

Here are a couple of highlights from what I read about them, both from the blog linked below:

"The larvae of Ellychnia corrusca, says the E.H. Strickland Entomology Museum, live in rotting wood and are predators, likely eating grubs and slugs that are also found in or on rotting wood.

Diet: Adults live on plant liquids such as maple sap, maple flower nectar, and possibly fluids from flowers during the fall before they find a winter hiding place, according to the E.H. Strickland Museum."

Here is a link that might be useful: What Lives in my Yard?

It is a great relief to know that the beetles under the bark of my oak tree are not a big concern.
Thanks everyone for all the input.