If I recall correctly, all Marine Corps officers must have at least a secret clearance. The Office of Personnel Management (OPM) will perform a background check before your package is even approved for your slot in OCS. If you don't disclose it you run a significant chance of not even getting your shot at OCS. I'm about to start drilling as a Recruiting Support Officer for the Baton Rouge, Louisiana RS. I'll be glad to answer any questions or assist you in any other way I can.