The phylum Proteobacteria has recently gained a new taxon Oligoflexia that represents the seventh or eighth (if yetto- be-validated “Zetaproteobacteria” is included) class, described by the only cultured species (as of December 2015), Oligoflexus tunisiensis, as the type species. This bacterium exhibits cellular polymorphism and presence of the uncommon fatty acid C16: 1ω5c as high as >65% of total fatty acids, besides its unique 16S rRNA gene sequence. The class Oligoflexia is characterized by the distinct phylogenetic cluster within the greater proteobacterial cluster, and certain environmentally-derived 16S rRNA gene sequences, a.k.a. environmental clones or phylotypes, of uncultured bacteria are now grouped into the Oligoflexia cluster; however, the content and extent of the cluster has not been clearly depicted. This mini-review illustrates that the Oligoflexia cluster hosts a variety of environmental clones from diverse sources. Currently 20 phylotypes (or clones) are affiliated with the Oligoflexia cluster, and the sources were ranging from soils to cyanobacterial mat, bio-filter, human skin, ant colony, desert, glacial ice, earthworm intestine, and seawater. However, their frequencies in respective clone libraries were generally as low as <1%, which indicates their corresponding species are only minor in respective microbial communities. Moreover, 61 environmental metagenome libraries yielded only 1198 partial sequences having >85% similarities (class-level affiliation) to the 16S rRNA gene sequence of O. tunisiensis, which accounts for merely 0.04% of those registered in Meta-Metagenomic Data Base (MetaMeta DB). On the other hand, >97%-similarities sequences were found in rhizosphere, and <95%-similar sequences were found from hydrocarbon-rich habitats such as petroleum reservoir. Thus, it is suggested that members of Oligoflexia may display cosmopolite distribution in general as well as endemism in certain geochemical settings.