"Cast Iron" plants are vigorous and hardy but don't flower and would probably do well. Dogwood may work but don't know if you want a tree under your oak.

Try azaleas again. Bring in some high quality potting soil, several bags for each bush, lots of mulch, and then water them every week, perhaps twice a week, all year long. They may need some extra water the next summer too.

I see bridal wreath under a neighbors oaks. Hostas too. The key here with a two hundred year old tree will be bringing in that potting soil. There is no danger of "smothering" the roots of a tree that size, and you dont want to damage the trees roots with your digging. Spend some money on the best potting soil available and plants will grow. Perhaps your oak has a few limbs that are in need of thinning. Most trees do. This will also help anything you plant below.

Dogwoods and redbuds make a great understory tree combination There is also now a whitebud form of redbud.