Oliverio, 70, is suspected of criminal association aggravated by mafia modalities.
He was elected with the centre-left Democratic Party (PD) in 2014 and held office until 2020.
The suspects also include former regional government members Nicola Adamo, 66, and Antonietta Rizzo, 59, as well as former regional councillor Seby Romeo, 48, all from the PD.
The raid was carried out by Carabinieri ROS special branch officers on Tuesday.
The suspects are being probed for various crimes including mafia-type criminal association and association aimed at fraud, extortion, unlawful competition and murder.
The ‘Ndrangheta is widely considered to have become Italy’s most powerful organised-crime syndicate thanks to its control over much of the cocaine that arrives in Europe.
ANSA