Many fans had been waiting and waiting for years to see how Oliver became a member of the Russian mob.
His status as a member of the Bratva was one of the earliest ways that Arrow hinted about Oliver not spending his entire five years alone on Lian Yu.
He may have returned to Lian Yu in order to stage a rescue and return to Star City with a vendetta against the corrupt.
Anatoly didn’t seem overly surprised to see Oliver back when they met up in the present in Season 2, so we can probably bet that Oliver wasn’t marooned there by the Bratva against his will.