my date of birth is 1998 and i done my plustwo in nios openschooling is me eligible for neet?

The minimum age to be eligible for NEET is 17 years as on December 31 of the year of examination. So if you appear for NEET 2021, by then your age will be almost 23. Right now, there is no upper age limit in Neet until Supreme Court passes its judgement on it. So if we consider that there is no upper age criteria, you are eligible as NIOS students are also treated as other boards students.
http://medicine.careers360.com/articles/neet-eligibility-criteria