"Silly Opossum: Ufo Attack!" originally titled "Orokana Opossamu Monogatari: Yūfō Kōgeki!" was a 4 part OVA released between 2002 and 2007. Little is known about its production as it received little coverage at the time and the OVA is not considered lost media. These Screencaps are all that's currently been found as of yet, though dedicated sleuths suspect has reason to believe there may be more discoveries in the near future.
