Here are some possibilities:

The majority of people can agree that oak trees are beautiful. But if they are left untreated, oak tree diseases will kill the tree. Many homeowners don't realize that their actions can contribute to a tree dying, so it is important not to make any changes to the canopy of an oak tree. Symptoms of oak tree diseases include discolored or spotted foliage, dead or dying foliage or branches, thinning canopy, shelf mushrooms or wet patches on the trunk. Although infections can lead to the death of a tree, one can treat them.

Phytophthora root rot is a disease that is soil-borne. This disease rots at the roots of the tree, and if the soil is warm or produces a lot of moisture, the virus can reproduce. Phytophthora can form if you water the oak tree beneath the canopy. Native oak trees thrive off of drought-like conditions. Another way that the tree can become diseased is if there is a buildup of soil or mulching material near the trunk. The disease thrives in this type of environment and is harmful to the oak tree's health.

Oak tree blister can be detected on the top and bottom of the tree's leaves. The surface of the leaves will look blistered, hence the name. On the other side of the leaf, it will look gray colored, but as the disease gets worse, it will turn brown. When the tree turns brown, it will curl up and drop prematurely. The tree won't die, but it will not be as full during the season as it should.

The most common type of diseases is Armillaria root rot, also known as oak root fungus. Oak root fungus is found in the root and tree trunks. If you see clusters of mushrooms growing at the base of an oak tree, this is a warning sign of oak root fungus. The health of the tree becomes compromised due to overwatering the tree, nutrient deficiency or excess soil build up. Interestingly enough, Armillaria root rot can last in a tree for over 20 to 50 years if left untreated.

Bacterial wetwood is the least damaging oak tree disease. Wetwood, also known as slime flux, is when an anaerobic bacteria infects the wood. The infected wood's composition causes disrupted membranes, high pH and low amounts of free oxygen due to small spaces in the tree being filled with water. Wetwood infects wounds, branch stubs, and root stubs, as well as alters the wood by causing high pH, high moisture and low oxygen due to decay-causing fungi. You'll notice the effects of wetwood because the tree starts to form a bunch of cracks called spider heart.

Other recommendations include, turn off the water. Don’t try to grow plants with high water demands under your oaks, particularly near the trunk. Even more important is to avoid frequent and shallow watering. You may have to choose between having healthy oaks or a green summer lawn in that part of your yard.

Protect the ground and tree roots from excavation and pavement. Avoid driving vehicles and tractors under the oaks. If you have an old oak in a new subdivision or where tree roots may have been damaged by nearby construction.

Majestic old oaks can add much to the value of your home and the pleasure of your yard. It is well worth understanding how to keep them healthy

Here are two publications you may find useful.

https://www.ces.ncsu.edu/depts/pp/notes/Ornamental/odin30/od30.htm

https://extension.psu.edu/oak-diseases

Hope this helps!

 From all of the research I did, this is likely a condition known as wetwood or slime flux.

•Wetwood causes discoloration of bark, sapwood and heartwood of many different tree species.

•Wetwood occurs when the wood of a live tree becomes water soaked and colonized by bacteria.

•In warm summer months, pressure builds in the affected wood and a foul smelling liquid, known as slime flux, is pushed out of the tree through cracks or wounds.

•As this liquid flows down the trunk, bark becomes initially stained dark black but eventually fades to a bleached tan or gray or black.

Even though the symptoms of this condition may seem unpleasant, the health of the tree is typically not affected.

Bacteria initially enter the tree through wounds on roots, trunks or branches. Protect trees from wounds by lawn mowers, weed whips and other equipment and use proper pruning techniques. Once a tree is affected by wetwood, there is no way to eliminate it.

Here is another publication.

https://palopinto.agrilife.org/files/2015/07/Oak-Tree-Diseases-and-Their-Control.pdf

 Hope this helps you.

Can you send photos of the entire tree and of the leaves?

Is this condition affecting the health and vigor of your tree?

What are the site conditions like? Does the tree have lots of space and sunlight?

Have you had any landscape changes, new plantings, or other activities within 30 feet of the tree in the last 5 years?

Maybe with better photos of the entire tree we can help you with your problem.

I hope this helps!

This could be the dreaded disease called sudden oak death.  Do these symptoms fit? Susceptible trees in the red oak group (coast live oak, California black oak, and Shreve’s oak) and intermediate group (canyon live oak) develop a bark canker when infected. External symptoms of canker development can include the bleeding of a thick, sticky sap. It typically smells like the inside of a wine barrel and is a deep burgundy but can vary in color from nearly black to an amber-orange . Recent rains can cause the sap to run, often producing large stains on the surrounding bark. Only larger trees—those that are more than 4 inches in diameter at chest height—show symptoms; infections of smaller saplings are extremely rare. Mosses and lichens growing on the tree trunk die if the sap comes in contact with them. Their death might be the only indication a tree is bleeding.

The bleeding is the external manifestation of an underlying, diseased area of the tree, or canker. Removing the surface bark will reveal discolored, brown tissue, normally separated from healthy bark by a distinct, black zone line, although this line can be somewhat indistinct during periods of active pathogen expansion, typically in the spring. This zone line represents the active front of the infection. Cankers usually develop 3 to 6 feet from the ground, although they can be as high as 12 feet or greater; they can be as low as soil level, but they are not thought to extend below the soil line. Bleeding sap initially appears on intact bark, without any obvious holes or wounds, although in later stages of the disease the bark might split.

Here are some publications to check out:

http://owic.oregonstate.edu/california-black-oak-quercus-kelloggii

http://ipm.ucanr.edu/PMG/PESTNOTES/pn74151.html