First, you will have to spend a couple of weeks studying and familiarizing yourself with the Chernobyl sewer system.
Oni has to stay on the move down there in order to avoid the rat plagues and to not arouse suspicion of the other locals. His drunken nomadic lifestyle causes him to release content quite slowly, but because the radiation is slowly altering his brain chemistry, his mind is beginning to transcend conventional human wisdom, and thus the perverted scrawlings he creates between phases of unconsciousness are high quality.
With respect to contacting Oni, we have found limited success with taping a few USD to a kielbasa sausage and soaking it in vodka for a half hour. Leave a few of those bad boys near the sewer grates by the old hospital and lie in waiting. He can be real greasy, but with a good pair of rubber gloves and a bit of patience you should be able to catch him.