Question
I have a western black oak in Oregon, that is about 100 + years old and am building a concrete slab about 10' from the trunk. I have heard that as a mitigation measure to preserve Oaks...you should lay some perforated pipes under the slab at ground level.... that are open to the air at one end and can let the roots "breath". Thoughts?

Answer
the problem is not the lack of air but that the roots are cut. Tree roots are within the first 6-18 inches of the ground. If you cut more than 20 % of the roots surface than you can expect the tree to have some dieback in the foliage. If you think of the tree as looking down and draw a circle around the tree where the limbs extent to and you cut through this circle like a pie and you remove or damage more than 20% of this "circle" then that is where the dieback comes in to play. It sounds like you will be much more than the 20%. The pipes will do nothing for the tree. When the digging for the slab is done this is where the damage occurs. If this slab is for a patio you might think of using a raised wood patio instead of the cement.
You Might try getting the advice of the Oregon State Forestry agency--they have Foresters who could come take a look and better help with the decision than I can without seeing the tree and the situation. Your city may even have an urban Forester who can help.
Here is the web page for the contacts in the Oregon Department of Forestry.
http://www.oregon.gov/ODF/offices.shtml