Multiple passes. On the table, with careful attention to keeping the piece against the fence. The stopped slot is pretty easy to do by pressing the piece down onto the bit, and during setup, mark the start and stop points on the fence. I'd use featherboards on the sides of the piece, and down onto it by mounting the featherboard to the fence. My own inclination is always to use a mortising bit for such cuts because it gives a nice flat bottom. With oak that's been around that long, I don't think it is going to warp.But see if you can set aside the material for an extra leg, just in case. You should probably use a forstner bit to drill into the leg to the depth you want for the groove so you don't have to press the piece onto the bit.