From the OCS website:
http://www.tecom.usmc.mil/mcub/library/MCUR/URCh1.htm#UR1005
c. Civilian clothes will reflect in style and quality the service "C" uniform. Non-inclusive examples of appropriate civilian attire:
(1) shirt with a collar/blouse,
(2) trousers/slacks with a belt (if applicable), and
(3) socks and shoes.
d. Examples of inappropriate civilian attire:
(1) beach or swim wear,
(2) gym or sweat gear,
(3) tank tops,
(4) short shorts,
(5) ripped or torn clothing,
(6) garments designed as underwear,
(7) clothing with designs of an obscene or suggestive nature, and
(8) shower shoes/flipflops. (ALMAR 123/96)
Polo style shirts in the summer; collared dress shirts in the fall and winter.