Please read TWIC Policies - USCG National Maritime Center
for TWIC requirements....and exceptions. As of Nov 2012, TWIC was required for new OUPV licenses. Similarly renewals without TWIC can take months longer than one with TWIC on file.
it appears that currently, NEW applicants for OUPV or those renewing that have never had a TWIC will need to apply/get/obtain one. All of the schools indicate that for first license grant, TWIC is required, subsequent renewals for OUPV will use the data currently in the TWIC system to validate background checks needed for the renewal.
IF you need access to secure port facilities, the TWIC is required regardless of license held. TSA does the TWIC, not USCG, the USCG uses TSA for background checks.
hope this helps, legislation has been proposed several times since the TWIC was created only to languish and die in committees of Congress, little chance of it being passed it appears this year.