It´s not very common that prequels exceeds its original, but this is exactly what happened with Ousama Game. OG: Kigen serves as a prequel and offeres a very unique insight to the times, where the "King" was forced to use letters instead of phone messages and how would a whole village react to the common enemy.
According to me, the story was very good and decent. Art was also very pleasurable. This manga contains totally different characters and I dare to say, better characters than Ousama Game. The explanation is given according to the whole "virus" thing and about creation of the "King" and "Kings´s Game" (aka Ousama Game).
I enjoyed this manga very much and definitely will reread it sometimes. For me solid 8/10 and I hope that other mangas in OG series will be just as good as this one.