No it's generally not an OC 955. It is usually a better binned CPU (works better than a 955/945). This isn't always the case, but the OEM (AMD) certifies that the CPU will work at given speed for at least 3 yrs. The 965/955/945 were all originally 970's, but some were downgraded to their respective speeds for one reason or another (don't run well at 970 speeds, need more 965's/955's/945's for sale, etc.). Intel does the same thing. All of their CPU's start out at the highest level possible when they start to fabricate them. When imperfections occur they will be downgraded to the best possible speed (outcome).
So a 965 might OC better than a 955, but most reviews that I've read, a 955 OC's about as well as a 965. Now I haven't read any reviews on the 970, so I can't comment on them, but I'm willing to suspect that they might be about the same when OC'd. The only difference is that with die shrinks, the newer CPU's (945/955/965) will OC a bit better than their predecessors.