https://ociservices.gov.in/
Foreign nationals cannot apply for OCI in India while on Tourist Visa, Missionary Visa and Mountaineering Visa. Moreover, the foreigner has to be ordinarily resident of India to be eligible to apply for OCI registration in India.
Note: 'ordinarily resident' will mean a person staying in a particular country or in India for a continuous period of 6 months
The first step to know is whats your status in India.. If you are on e-visa/tourist visa like mentioned above you cannot apply for OCI in India..
If you have an Entry Visa (x) which is issued to former Indian nationals then you might be eligible to apply only after living in India continuously for over a period of 6 months..