You don't mention where you are, what kind of oak tree, or how long this has been going on.
Generally oak trees don't need supplemental water-sprinklers, hose, etc. once established. That might be two or three years.
Watering to the point of saturating the ground and keeping the ground saturated can cause rot. The tree would eventually die from this.
I'd have a certified arborist take a look at the tree and if he/she sees anything harmful in the present conditions get a report written up. Go over the report with your neighbor and see if they will cut back on the watering. If you live in a community where there is a homeowners association you may be able to present this to the board and have them put pressure on the neighbor to limit watering. If the tree was saved when you home was built because it adds to the property value of your home you may want to look into a small claims court action for an injunction regarding the watering problem.
If the arborist determines that the overwatering problem has damaged the tree and then the tree dies you maytake your neighbor to court for devaluing your property-every state is different so see if you can get a short, free consult with an attorney.
What ever, take photos of the tree as time progresses and photos of the saturated soil too. The photos could become evidence in court.
