Dear Sheetal,

In order to take admission in NITs for OCI holder you have to appear for DASA.

Undergraduate engineering admissions to foreign nationals, Non-Resident Indian (NRIs), Persons of Indian Origin (PIO) and Overseas Citizen of India (OCI) is offered through DASA.

It is expected that application form of DASA 2018 will be tentative out in the first week of April 2018. 

To know in detail you can refer below link

https://engineering.careers360.com/articles/dasa-ug?icn=dasa_article&ici=dasa_exam_rdmrvp

Good Luck!!