I was woolgathering about the notion of a scheme, and it occurred to me that I know of no scheme $S$ that has an open subscheme of it the ring of integers $O_K$ of some number field $K$.

It would be interesting notion if one could patch rings of integers together to form some non-affine $1$-dimensional normal scheme $S$. The fact that I've never seen an example makes me think it's impossible.

###Question

Is it true that if $Spec(O_K)$ (the ring of integers of some number field $K$) is an open subscheme of a scheme $S$ then it is equal to it?