Oliver is most likely based on the real life chimpanzee Oliver. Oliver was thought to be a missing link or even a human-ape hybrid because of his human-like face and his tendency to walk upright. These theories had been abolished when several exams done to him confirmed that he was just a regular chimpanzee.
"A mysterious bipedal creature that was much talked about in the mid 70s. He has made an appearance in Japan. Since his discovery, there was much speculation about whether or not he was was a mix of chimpanzee and human. It was said that he had one less chromosome than a normal primate, but, in the end, he was determined to be a regular chimpanzee."
—Devil Summoner: Soul Hackers 3DS Compendium
Oliver is the lowest level UMA, and is only unique in that it can learn Infinite Taunt, an ability that greatly lowers the defense of opponents.
In the 3DS remake, a variant called Oliver Zero is available from Nemechi, one of the first available from the start. He costs 100 D-Souls and uses the original artwork from Shin Megami Tensei: Devil Summoner. This version is a lot more sturdy and gains healing abilities in addition to Infinite Taunt.