Roast beef was New Orleans' most popular po'boy filler up to the 1970s and fried oyster po'boys are popular enough that they are sometimes called an oyster loaf, but the fillings can be almost anything, according to Sarah Rohan who in her book "Gumbo Tales" mentions fried shrimp, catfish, crawfish, Louisiana hot sausage, fried chicken, baked ham, duck, and rabbit.