Candidate must have done 10+2 from any recognized Central or State Board or its equivalent. Candidate must have scored at least 75% aggregate in Physics, Chemistry and Mathematics and 60 marks or above in each individual subjects. English must be compulsory subject.
So, you are eligible or the BISTAT exam after pursuing 10+2 board exams from National Institute of Open Schooling as it is central government approved institute for conducting board exams.

NIOS certificate is legally valid and NIOSboard is equivalent to any other educational board in India. No institution should reject a candidate who has met all other admission criteria based on his Nios certificate along.
You can check it here:
http://www.nios.ac.in/media/documents/listofuniv.pdf

In order to apply for BITSAT Entrance Exam, You must have secured minimum 75% marks in aggregate in Physics, Chemistry and Mathematics/ Biology in your qualifying exam or equivalent.

Those candidates who have passed 10+2 or equivalent exam in 2017 or earlier will not be eligible for BITSAT 2019.

You can check the exam eligibility with the given below link.

BITSAT Eligibility

Good Luck!