Oklahoma, USA is known to have low seismicity, based on historical records as well as recent instrumental records. For enhancing oil production, fluids were injected in deep borewells. An increase in seismicity was observed after about 20 years of initial fluid injection and about 5 years of the substantial increase in wellhead pressure including occurrence of M 5.7 and M 5.8 earthquakes in 2011 and 2016. Subsequently, fluid injection was curtailed. An investigation of the characteristics of the fluid injection induced earthquakes revealed that fluid injection induced earthquake sequences have characteristics like RTS. However, with so many wells operating simultaneously, the observed seismicity appears like earthquake swarms. The chapter shall deal with the fluid injection induced/triggered earthquakes in Oklahoma, USA.