An 11-member CBI team questioned OMC staff at its office and later visited Reddy's house. Janardhana Reddy has been asked to be available for questioning.
The Centre has ordered the CBI probe into alleged illegal mining at the Andhra Pradesh government's request.
Reddy, who has a case pending against him before Supreme Court, is accused of illegal mining in parts of Karnataka bordering Andhra Pradesh. Reddy holds three mining licences in Andhra Pradesh.
Soon after Andhra Pradesh sought a CBI probe, opposition Congress and JDS in Karnataka stepped up pressure on the state government to seek a similar probe, which Chief Minister B S Yeddyurappa flatly rejected.
Yeddyurappa has maintained that there is no illegal mining in Karnataka and he has not granted any new mining licence ever since he assumed office.
The Supreme Court had appointed a Central Empowered Committee which concluded that OMC was carrying out "illegal" iron ore mining on the Andhra Pradesh-Karnataka border.