I drew a circle on the hearth, and laid out the first ring. The hard way is to lay out the desired opening (in my case, 20 inches) and then start at one edge of the opening and lay bricks all the way around to the other edge. The easy way is to lay the first brick in the center, at the rear of the oven, and then lay bricks on both sides until you get near the opening. The final bricks can then be cut to fit. I’ll do my best to keep all of the cut bricks out of view.
I also did a little thinking about how to create the opening to the oven. Irrational or not, I’m against using angle iron inside the oven. Therefore, an arch is called for. Actually, several arches. The first arch will be the opening into the oven chamber, and will incorporated into the oven wall. The second arch will be the vent area, and it will have a slightly larger radius to create a reveal, so a door can seal the oven.
Getting a feel for the width of the opening:
This bricks will be cut to form the arch for the opening into the oven: