According to the Korea Times, the probe has begun after the case was referred to the Seoul Central Prosecutor's Office by the Financial Supervisory Service (FSS) at the end of June.
It is claimed that executives at a daily newspaper in the country and the son of the incumbent prime minister used information not available to the public in order to make a profit by trading in OCI.
"We've summoned two FSS officials as an initial step for the investigation," a prosecution official was quoted by the news source as saying.
Kim Jae-ho, chief executive officer of the Donga Ilbo newspaper, has been identified as one of the suspects, along with the son of prime minister Han Seung-soo.
Earlier this month, OCI refuted claims that its chairman, Lee Soo-young, was being investigated for potentially having a role in the alleged crime.