See 46 CFR 11.201(j)(1)(i):
Except for an endorsement required by the STCW Convention, any applicant for a deck or engineer officer endorsement limited to vessels less than 200 GRT, or an officer endorsement limited to uninspected fishing industry vessels, may request an orally assisted examination instead of any written or other textual examination. If there are textual questions that the applicant has difficulty reading and understanding, the Coast Guard will offer the orally assisted examination. Each officer endorsement based on an orally assisted examination is limited to the specific route and type of vessel upon which the applicant obtained the majority of service.
Until March 24, 2014, the threshold was 500 GRT.