We don't know which species of oak you have either, but this USDA Plant Profile map shows that Quercus palustris (Pin oak) does grow natively in Mercer County, at the "bend" on the western side of New Jersey. We will do a little research and try to determine some factors that might be causing the leaf drop, including the age of the tree.
We found a GardenWeb Forum in which your precise question was asked. Because we couldn't think of a better way to say it, we are going to quote one of the answers, which was about an oak tree from Pennsylvania, right next door to you:
"We took down a huge red oak last year, both my wife I cried the day they came with the chainsaws. The tree had a fungal infection, which was confirmed by sending the fruiting bodies, those "mushrooms" that were growing around the base of the tree, to a lab for analysis. Usually, a fungal attack kills a tree from the inside out and can take years and years to occur. What you have to remember is that trees distribute water and food through their outermost layers of trunk, the phloem and xylum. What that means is that a tree can look perfectly fine on the outside, while the inside is all but gone. And this makes for a dangerous situation. It's what arborists call a "hazard tree."
A word of caution - be careful about whatever "tree man" you use. Some are very knowledgable, while others are just guys with a chainsaw and a truck. We had several certified arborists examine our tree, and what we found was that fixing it was very expensive, and came with no guarantee. We were told that if this tree fell on our house, it would destroy the attic and pretty much all of the second floor. Not a very comforting thought. They all seemed to favor taking it down to be 100% safe, and so we did.
Sorry to lay out what might be too much info here, but we went through the process over the last couple years, this was after what was deemed a healthy white ash blew down in a thunderstorm and destroyed a brand new vehicle. Please understand that we are big tree lovers. However, we have also come to realize that some trees, meaning big trees with problems, are very, very dangerous."
From hundreds of miles away, Mr. Smarty Plants could not possibly diagnose the problem nor recommend a treatment, if there is one. We strongly urge you to get a trained, licensed arborist to look at the tree. You will have to pay him for the advice, but that way, he has nothing to gain one way or the other, and you can trust to the honesty of his recommdendation. Considering the storms you have had in New Jersey recently, we would fear the consequences of an old tree weakened by some infection more than the loss of the tree. If it fell in a storm, you would not only have lost the tree but also whatever else is destroyed, possibly including yourself.
This member of the Smarty Plants Team is personally very sympathetic. When we built a house in Texas over 50 years ago, the property was populated with post oaks. One, in particular, our arborist estimated was 250 years old - older than our country! We lived in that house, and I cherished that oak, for 38 years. Some years later, someone told us that a swimming pool had been built in the back yard, causing many of those trees to be destroyed. Some things are simply out of our control, but we still mourn that tree.