A registered Overseas Citizen of India (OCI) is granted multiple entry, multipurpose, life-long visa for visiting India, he/she is exempted from registration with Foreign Regional Registration Officer or Foreign Registration Officer for any length of stay in India, and is entitled to general ‘parity with Non-Resident Indians in respect of all facilities available to them in economic, financial and educational fields except in matters relating to the acquisition of agricultural or plantation properties’. However, they require special permission from the Indian government to conduct research in India.
This permission is obtained by applying online through the Online OCI Services portal (www.ociservices.gov.in by selecting “research activity” and uploading the required documents, which typically include a bonafide certificate from an Indian institution, passport and OCI card copies, and a research synopsis.
Please find the detailed instruction in the linked pdf file