Technical Details to consider when designing an oven of any type.
1. Airflow for the fire is primarily concerned with exhaust flow. The fire produces heated air that needs to be able to move away from the fire. You need a chimney at least 2 block wide to allow faster moving exhaust to pass slower moving exhaust or your fire will smoulder.
2. Fires need air blocks around them on at least one face.
3. Air around objects you are cooking will cool them off. This includes the corners around the object being cooked. Fill empty space with slabs or blocks to prevent the item being cooked from cooling down while cooking.
4. Slabs are treated like full blocks - They don't allow air and they block heat transfer. They allow light and you can reach through them to put stuff in or take stuff out of your oven. The lever can be used to rotate slabs to provide the desired effect.
You can make heated air visible in game by setting hot_air in the fire nodes meta data. This can be done in testing using using a mod like alternode to edit the fires meta data during test. Causes it to create a climate:air_temp_visible instead of climate:air_temp node. Very useful for troubleshooting heat issues. Not intended for game play.