Armillaria root rot is common in landscape plantings of oakleaf hydrangea. The initial symptom is wilting of one or more shoots that previously seemed healthy. Irrigation does no good and within a few weeks the rest of the branches wilt and the plant dies.
Poor drainage is usually the culprit when this root rot attacks. If less than half of your hydrangea is affected, remove the dead limbs, dig it up, add plenty of soil conditioner to the planting area and replant. With a bit of luck it may send forth new sprouts and live to bloom another day.
My colleague Theresa Schrum observed sudden wilting on her oakleaf hydrangea. When she investigated the base of her plant, it was evident that root rot wasn’t the culprit. Voles, the rodent rascals, had gnawed off the bottom of the hydrangea stems. No wonder it suddenly looked so poorly!