Bacterial wetwood or slime flux is a condition that occurs when any of several species of bacteria enter the tree, usually through wounds in the roots, since many of these bacteria are soilborne species. The bacteria infect inner sapwood and/or heartwood and produce sour-smelling gases and liquids through their fermentation processes. These fermentation byproducts build up inside the trunk creating tremendous pressure, and find their way to the tree surface, usually through a wound in the bark. Although this condition cannot be treated or cured, in most cases it will not kill the tree but can reduce its vigor, and it sounds like your oak may be in a state of reduced vigor. You can help your tree to some degree through reasonable management strategies: watering during drought, protect the trunk - keep string trimmers away, spreading 2-3 inches of mulch on the ground around the trunk (a 6 foot circle, but never piled up against the trunk) depending on size of tree. You could apply a half- to 1-inch layer of fully finished compost, gently rake it into the grass under the tree and then lightly water the compost to settle it onto the soil surface. Compost contains much beneficial fungi and soil microbial life that will likely benefit the tree. At some point, since the condition is not curable and your tree sounds like it's otherwise struggling, you may consider having an arborist look at the tree for potential removal. Additional information on this condition here:

http://kentcoopextension.blogspot.com/2009/08/landscape-wetwood-and-slime-flux.html

http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/cankers/bacterial-wetwood.aspx

http://www.mortonarb.org/trees-plants/tree-and-plant-advice/help-diseases/wetwood

-NYBG Plant Information Service