WSO is a worldwide government with access to some of the most cutting-edge advanced technology. Not much is known about it other than it controls the Skull Force military group and sends them on several threatening missions. The only named member of WSO is the Skull Force Commander Kojindani.
According to the prequel novel "Project Kaiser", WSO stands for "Wide-area Surveillance Organization". The organization also appears in the multimedia series "Seelen the Hunting World".