The brown leaves/branch tips might be due to recent egg-laying by the periodical cicadas. The female cicadas inserted eggs into branches (oaks are one of their preferred species and damage to oaks has been widespread). The insertion of the egg "nests" can interrupt water flow, resulting in the brown, dead tips (referred to as "flagging"). This is not considered harmful to a mature tree and is a form of natural pruning. 

More extensive dieback symptoms (entire branches with no leaf development) are typically a symptom of a root or trunk problem. It looks like there is a frost crack on the trunk of your tree, but it looks older and healed over - not a big cause for concern. 

The branch on the upright right side of your first photo appears to have epicormic shoots (numerous small sprouts coming up from the main branch). This is typically an indicator that there is some type of wound/injury. 

As a next step, I would encourage you to consult with a certified arborist who can evaluate your tree on site. You can find an arborist using this online directory from the International Society of Arboriculture: https://www.treesaregood.org/findanarborist/arboristsearch.

Christa