Security clearance
Back in 2005-2006, a pilot friend of mine with a US passport was able to sit for the DGCA exams, without needing to get any security clearance.
Have things changed since then? Because now, if you hold a foreign passport and wish to take the DGCA exams, you require an NOC from the DGCA, which is obtained after getting a security clearance. This even applies to folks with an OCI.