In a deep braising dish add the olive oil and cook over low heat for several minutes, then add the pork loin. Add salt and pepper and cook until the meat is golden brown on all sides, turning a few times. Then add the wine and cook until it evaporates.
Meanwhile, cut the onions into 8 slices each. Cut 2 oranges (leaving the rind on) into 8 slices each. Juice the other 2 oranges and filter the juice to remove pulp and seeds. Add the orange slices, orange juice, and onion slices to the meat. Then, add bay leaves and more pepper. Allow it to cook over low heat, covered, for an hour and a half. While cooking, if the meat becomes too dry, you can add water or broth.
When it is cooked, set aside for at least 10 minutes before slicing into thin strips and arranging on a large serving dish. Then place the orange and onion slices around the meat slices, drizzling the pan juices onto the meat.