Judging from the lid, this is a #16 oven.
If you would like to restore it, first give it a good scrub down with soapy water and a stiff bristled brush. After that, I’d look for any damage like cracks. It might not be worth restoring if there is any serious damage. If there is still any organic matter on it, you’ll need a lye bath to remove that. Then it will need a whole lot of electrolysis and continued scrubbing until you can get down to bare iron. A #16 is a big and heavy piece. You will need a good sized electrolysis tank to hold that job.
It can be done, but needs serious work.
I wouldn’t be surprised if pieces of iron actually flake off from the oven in the electrolysis tank.