Is my bur oak dying or just stressed?

Bur oak declines slowly and rarely from anything species-specific, so when one thins out, look at the root zone first — it usually means fill, trenching, or compaction rather than a pathogen. The two named disorders worth ruling out are bur oak blight, a Tubakia fungus producing purple-brown wedge-shaped lesions along the leaf veins in late summer with dead leaves clinging into winter (documented in isolated Midwest locations including Illinois, not widespread), and bacterial leaf scorch, which browns leaf margins and returns worse every year with no cure.