Dear Aspirant 

First of all Congratulating you on your nice performance and I would like to tell you that if you belong to or passed your class 12th from national institute of open schooling then definitely no body or institution can deny you from taking admission, so you don't have to worry about these things.

Good luck for your future :) 

Feel free to ask if you have any further queries. 

Hello,
Definitely You are Eligible to appear in the IIT JEE Entrance Examinations. So, if you have Completed Your 12th Then You are Eligible to appear In IIT JEE Examinations.
Because it is Clearly Mention that If the Student can Pass in Senior Secondary School Examination conducted by the National Open School with a minimum of five subjects are eligible to appear In the IIT JEE Entrance exam and its counseling as well.
To know about the Jee main eligibility criteria, you can go through the below article -
https://www.google.com/url?sa=t&source=web&rct=j&url=https://engineering.careers360.com/articles/jee-main-eligibility-criteria&ved=2ahUKEwiTmdbZqqTjAhWEfH0KHYkeDpsQFjACegQIDxAI&usg=AOvVaw2vh8y3vHasXxXeWyutFhSj

Hope it helps