As I am sure you know, FACs flew various different aircraft during Vietnam and came from the various services as well as our Aussie brethern. Since you asked about O-2As, most of the photos I have seen with survival gear have the mesh vest with the various designed or modified pockets on them. Not to say that other services might have worn different, but you did ask about O-2 jocks.
As for helmets, most of the photos I have seen show crew wearing the single visor helmet (the device to lower or raise it in the center) and a single boom mike. Although I have seen photos of pilots wear soft head gear, I am not sure if that was something worn in lue of the helmet in flight.
If you are looking for a photo heavy book on FACs in Vietnam, Hit My Smoke, Forwar Air Controllers in Southeast Asia by Jan Chuchill is a good one. Although not photo heavy, I highly recommend Naked in Da Nangby Mike Jackson as a great book on a O-2 pilots tour of duty in Viet Nam.
Here are two on line sites that might help too:
Last edited by AirCommando; 03-03-2008 at 13:15.
Reason: Added web sites
"I'm so used to being the weirdest guy in the room, over here I am not even in the running" Billy Covington (Robert Downey Jr, Air America)