We identify by ab initio calculations an orthorhombic carbon (O-carbon) in Pbam (D-2h(9)) symmetry for compressed graphite in AA stacking, which is formed via a distinct one-layer by one-layer slip and buckling mechanism along the [210] direction. It is dynamically stable and energetically more favorable than other known compressed graphite phases, albeit its slightly higher kinetic barrier. The O-carbon is comparable to diamond in ultralow compressibility, has a band gap wider than that of diamond, and is compatible with experimental x-ray diffraction data. The present results offer insights for understanding the complex structural landscape of compressed graphite and the versatile nature of carbon in forming a rich variety of structures under pressure.

http://dx.doi.org/10.1103/PhysRevB.85.033410