I have an area under a large oak tree where I would love to plant

Since there will most likely be considerably shade, I would suggest something like pachysandra. It is very hardy.

Hostas are a good option too

Bulbs

You will need to be diligent with watering if planting under a large tree. The tree will block both above and below the smaller newer root systems from obtaining water. The best way to do this would be with a drip irrigation system and a shallow mulch layer. Great shade plants, ,, perennial Geraniums, toad lily(picture 1) Columbine (picture 2) Astilibe (picture 3)

Hosta (picture 4). Huechra-Coral Bells (picture 5)

Hostas work