Neat. It's what's known as a 'doublet concretion'. Concretions in sedimentary rocks grow around a nucleus (sometimes piece of plant or shell debris) by successive precipitations of minerals before the surrounding sediment fully lithifies. If it so happens that two nuclei are relatively close together in the sediment and their respective concretions become large enough then they will ultimately conjoin to give you that shape. They're commonly formed in sandstone where iron-rich groundwater has percolated through it. That's an unusually large one.
I live it Oklahoma and my grandma must have found it here. It was in my grandmothers things when she passed. It could possibly be a grinding stone for a conuchi stone. I also have that. My grandma was full blood creek Native American.