Natural fission reactors at the Oklo uranium deposits in Gabon appear to have formed in a short interval of geologic time during which uranium could migrate to form deposits and the 235 U/ 238 U ratio was still high enough to trigger fission reactions. At the time of sediment deposition in the ore-hosting Franceville Basin ∼2100 m.y. ago, the oxygen deficient atmosphere would have inhibited uranium dissolution and therefore its migration to form deposits. Dissolution and migration of uranium probably began only during later diagenesis after ca. 2050 Ma, and local reduction reactions in the presence of hydrocarbons allowed formation of high-grade uranium deposits. At this time the 235 U/ 238 U ratio was still significantly higher than it is today, thus triggering nuclear fission reactions. Before 2.0 Ga, the 235 U/ 238 U ratio was also high enough to allow fission reactions but no mechanisms were able to produce high-grade uranium ores. Thus, oxygen in the atmosphere was probably the main factor controlling the occurrence of natural nuclear fission reactions. This conclusion is in agreement with earlier suggestions that oxygen contents in atmosphere increased during a “transition phase” some 2450–2100 m.y. ago.