I wanna post my guess before the experts.

This may be somethig called bacterial wet wood. I THINK moisture in the tree gets squeezed out in a quick freeze. It is not a death sentence but a reminder to have your tree looked at by a pro sometime.

There or someplace upwood from there water has probably been getting in for a long time. I had a split in a crotch in an ash tree causing a problem which LOOKED similar to yours. As the tree iverhung my Mustang's garage it jad to be dealt with.

Guys, am I close?

Thanks for the response. You may be right
Waiting on a tree guy to call me back.
Just had a ton of snow and its in the teens here right now
so no ones rushing to go out right now.

i dont know if sap is running that fast in NJ ... this time in winter ....

i would be looking for an open wood in that crotch above ...

is your tree guy a ASA certified arborist...

or a monkey with a chainsaw ...

the issue is interior rot.. and whether this tree is hanging over anything and threatening it ... should it fail catastrophically ... in a storm, lets say ...

i would think.. you have plenty of time to figure it out.. with an oak ... so try to relax in the mean time ... there really isnt going to be anything you can do about open wounds .... other than take down the tree ... [and i mean weeks or months.. not decades]

ken