Hello Aspirant !

As , you had not mentioned the specific open school for which you want to know , I will give you a brief idea about open school admission in NIOS.

Admissions are open for 12th class in open school for NIOS ,  you can apply online . Last date for admission is

30th September (Without Late Fees)
From 1st October To 15th October (With Late Fees)

You can also refer to these links for more information regarding dates :

https://school.careers360.com/articles/nios-12th-registration

https://sdmis.nios.ac.in/

Hope it helps !