There are a couple exceptions, but basically you have to roll a 5 or less with a single die to gain concealment in OG. This is within 16 hexes of an unbroken enemy unit but out of LOS, or at 17 hexes or greater if in LOS.
The main DRM are US# (Unit Size #) and leadership modifier of any leader present. Stealth is a -1 and lax is a +1. The way I remember US# is the number of soldier silhouettes on the counter - a squad has 3 soldiers on it, so the US# is 3. A HS has 2 soldiers for a US# of 2 and a SMC has 1. Large guns or vehicles are a 5 (the biggest US# that you could possibly gain concealment with a dr of 1), all other 5/8" counters are a 4. Just add the US# to the die roll, if you get a 6 or better they can't gain.
The concealment loss/gain table is your friend. There is a table in the lower right corner on the same chart page with the concealment loss/gain table that lists all the other DRM for gaining.
cheers
