Chris is quite right, and if you look at pictures of the 2 breeds today you will see some clear differences in shape. Orps are just bigger, fluffier, and more rounded in shape, and are much less what we call 'cut away' in the front. The tail carriage is quite different on the females in particular, an orp has quite a voluminous tail, almost cushiony, whilst the australorp is more upright. The australorp is a much better bet if you are looking for egg production in the females. But to my very biased eye, there is no equal to the female orp for beauty!