Ah, the theoretical Oort Cloud. As with the mythical Quark - give it a name and it becomes real. It is not real. Humans assume that comets just happen along, and sometimes get caught in a repeating cycle. Humans speculate on the origin of comets. The majority of comets visiting your Solar System return because they originated here, during planetary breakups caused by the periodic passage of the 12th Planet, for one. Look to the nature of your Solar System, the composition and shape of the planets. How would irregularly shaped chunks of ice, which in essence comets are, begin? The Earth was once a water planet, but lost much of her water following a collision with a traveling moon of the 12th Planet. Where do you suppose her waters went?