You can always put a smaller oven in the cabinet, though you may have to work with building a support frame for it. The oven panel is cut to fit closely around the front of the oven, behind the oven trim.
I've got a 27" oven in my 30" wide oven cabinet:
The inside width of the 30" oven cabinet is IIRC 28-1/2" wide. Some 30" ovens may need some adjustment of that width to fit - there has been discussion here of ovens that will or will not fit without work on the cabinet, as well as discussion of how to support heavy ovens.