Wall oven smaller than Cabinet Space
I would buy some wood of the same species as the cabinets. (probably 1 x 4s) Then build a frame out of it that will end up being exactly the same width as the oven cabinet in outside dimension, and the correct size for the oven in inside dimension. Use biscuits, glue and clamps to attach the frame pieces together. (I am assuming you dont gave a Kreg Jig pocket screw kit). While that is drying, cut out the existing face frame of the cabinet to accept the oven. After the new face frame is dry and stained to match your cabinets, hold it in place over the existing cabinet. When you have it lined up where you want it, C clamp it in place and screw it to your existing cabinet from the inside (reaching into the hole) using inch and a quarter long drywall screws or preferably deck screws. You will want to build up a floor inside the cabinet that the oven will rest on...you dont want it hanging from your face frame.
Hope this helps.