Olduvai Hominid 5 (OH 5) is easily the most famous of the early human fossils found at Olduvai Gorge. It is a nearly complete cranium of an adult male P. boisei. Originally named "Zinjanthropus boisei", its classification was changed to Australopithecus boisei, placing it in the same genus as the southern African species. The fossil is still referred to today as "Zinj", in reference to its original name.

The South African Paranthropus robustus provided the original standard for the robust cranial form: a large sagittal crest on the top of the skull, a flat face formed by large zygomatic arches positioned far forward and "megadont" cheek teeth. With the discovery of "Zinj" in eastern Africa, a new level of robusticity was defined, sometimes called "hyper-robust". Notice the wide zygomatic arches which project forward of the nasal opening and form the dished-shape face typical of Paranthropus boisei. The outward flaring of these bony arches from the side of the head provided space for large temporalis muscles. These were the huge chewing muscles that passed from the lower jaw to the large sagittal crest atop the skull. In some cases, the megadont cheek teeth of Paranthropus boisei were four times the size of our own (see the photograph at the bottom).