I'm not sure if this is true, but I found forums where people have wrote this:
"Oleg was born with 6 fingers but had one cut off at a young age to fit in.. Result of the chernobyl fall out. Mutated left arm is much stonger and muscular than it should be. Literally freak of nature plus radioactivity."
people say he grow up near Chernobyl. I think he is too young to have been alive at the time of the incident but the area is still contaminated.
This could be false but it would make sense because that forearm disparity does not look natural.