Hey Tommy,
I'm back in the office.
I think you are talking about the "arch" into the wood store under the oven. You can build one of those as a decorative, not structural effect. You use the angle iron in the stand, then finish the opening with an arch. If I had an oven to build right now, I would do the stand that way.
As dmun noted, you can build the opening into the oven chamber itself as a brick arch. The Artigiano is built that way, and a handful of Pompeii builders have done that. The arch into the Casa oven chamber is naturally curved.
James