Olivine and low-Ca pyroxene compositional distributions show a hiatus between H and L, but not between L and LL. Because H, L, and LL chondrites show systematic changes in many characteristics, they must have formed in close proximity. H/L and L/LL chondrites may be anomalous members of one of the major OC groups or representatives of OC bodies of intermediate composition. A few highly reduced OC are either H chondrites that underwent whole-rock reduction or are members of otherwise-unsampled reduced OC bodies. IIE irons likely represent a fourth, reduced OC group. R chondrites resemble OCs but have more matrix material, higher 17O, and are much more oxidized. H, L, and LL chondrites show increasing degrees of oxidation with petrologic type. The bulk chemical and bulk isotopic compositions of OCs show systematic variations among the four principal groups. Metal-silicate fractionation was a nebular process that may have been caused in part by loss of metal from chondrules. OC oxidation state is heterogeneous on global and kilometer-size scales, and homogeneous on meter and smaller size scales. OC bulk O-isotopic composition is heterogeneous on global size scales and homogeneous on km and smaller size scales.