Hey,

Yes, open school candidates are eligible to appear for NEET entrance exam. National Institute of Open Schooling (NIOS) is recognized at par with other prominent educational boards in India. The only eligibility criteria an Open school candidate is required to fulfill are: 

• He/she should clear Class 12 or equivalent exam with atleast 50% aggregate marks in Biology, Physics and Chemistry.
• Candidates should have completed 17 years of age in the year in which they take MBBS admissions.

The only disadvantage NIOS students will face as per the latest MCI guidelines is, they are not allowed to pursue MBBS abroad as they will not be issued an eligibility certificate because it requires classroom training. Apart from that, there is no restriction on NIOS students to appear for medical college admissions in India.
Hope this helps.