meatburner, Welcome to the Smoking Meats forum. We have a wealth of knowledgeable folks here in the forum. Let us know your set up and we can help solve the problem.
In order for us to help you we need to know the type of smoker that you are using, the fuel source (ie: wood, electric, charcoal or propane), and if the wood you're using seasoned (dry) or green? You mentioned oak firewood so I'm thing that this wood has been dried.
Along with over smoking and using green wood, another cause of bitter tasting food that plagues folks new to smoking, is not understanding how to use the intake and exhaust vents properly to control airflow and heat. Some believe that by opening the exhaust vent they are letting all the heat escape and closing it will keep the heat in and help them maintain temperatures. Closing the top/stack/exhaust vent traps the smoke inside the smoker and stops the airflow causing a few problems. With the flow of air cut off the fire will begin to die down and temperatures start to cool, at the same time the smoke is trapped in the cooker and begins to cool. This cold trapped smoke quickly goes stale and will cause a bitter, acrid takes to the meat that is being cooked.
The best way to prevent this is to leave the exhaust vent half to full open (all the way open is best), keep the fire small (just big enough to do the job) so you don't need to close the vents for temperature control, and use the intake vent on the fire box to control temperatures.