Thanks for the question.

The two most common diseases among oaks in Minnesota are oak wilt and oak anthracnose. Of these two, oak wilt is more serious. Here is some information about both including their symptoms:

https://www.dnr.state.mn.us/treecare/forest_health/oakwilt/index.html - :~:text=Oak wilt is a deadly,fagacearum, formerly Ceratocystis fagacearum).

https://www.dnr.state.mn.us/treecare/forest_health/oakwilt/identification.html

https://ag.umass.edu/landscape/fact-sheets/oak-anthracnose

https://yardandgarden.extension.iastate.edu/article/2006/5-24/oakantrhacnose.html

Of these two possibilities, I suspect that your oak is showing signs of anthracnose.

If you really want to nail things down, you could do one (or both) of the following:

1). Enlist the services of a professional arborist for a compete inspection and assessment. See:

https://extension.umn.edu/planting-and-growing-guides/how-hire-tree-care-professional

2). Consider submitting plant samples to a diagnostic disease laboratory for an analysis. This would be a definitive way of getting to the bottom of things. Both Wisconsin and Minnesota have excellent labs to do this. Before submitting anything to either of them, call first as to what type of samples should be submitted. Both have a nominal charge for their analysis. See:

https://pdc.umn.edu/

https://pddc.wisc.edu/clinic-overview/

Good luck. Thanks for using our forum.