OMA is a database of orthologs extracted from available complete genomes. The orthology predictions of OMA are available in several forms:OMA Pairs: for a given gene, a list of predicted orthologs in other species is provided. OMA Groups: a set of genes across different species which are all orthologous. OMA Hierarchical Groups: the set of all genes that have evolved from a single ancestral gene in a given taxonomic range. OMA Genome Pair view: the list of all predicted orthologs between two species.