This is going to be a dry shade condition - a tough situation for most flowering plants. The Solomon's Seal should work, but the toad lilies prefer a moister situation and hellebores actually prefer quite a bit of sun. Epimediums would work in this situation, but is not a heavy flowering selection, with wands of small, orchid-like flowers in early spring. Fernleaf dicentras (bleeding hearts) are a good choice as are Geranium nodosom and macrorrhizum. Hostas are actually very drought tolerant once established and they can offer a lot of color variations even though the flowers may not be as showy as you would prefer. Start with small plants to avoid disturbing the oak roots unnecessarily.

tiarella