Abstract: This invention relates to histo-blood group O alleles of baboon. The major O allele retains identifying sequence characteristics of the baboon A allele. Other alleles, including a B-like allele, are also disclosed herein. The O alleles of baboon are closer in sequence similarity to the baboon A and B alleles than are any of the baboon ABO alleles to the human alleles of the ABO locus. This invention also provides materials and methods for identifying O alleles, and carriers of O alleles, as well as methods for producing baboons and baboon cells, tissues, and organs having a group O phenotype. Group O baboons, cells, tissues, and organs of the invention are useful for xenotransplantation.