Well as per Govt Rules (refer to ESTABLISHMENT Code or ESTA Code), a permanent government employee have to obtain NOC to apply on higher post in any other government organization!

Mostly advertisements have closing date of 15 days, you can apply for NOC in your office before closing date.

When applying for new post in other organization, mention these words on form "ADVANCE COPY" and if there is space on form, write these words "NOC will be provided at the time of test/interview"

It is prudent to mention that as per ESTA Code, a permanent govt employee can only have 2 x NOC's per YEAR!, don't know what is happening in your office but its best if you mention your organization's name here so someone may guide or you can contact your organization's HR to know about the rules in vogue/practice there.

and YES, you have to get NOC every time you apply for a new Job!

Regards,