apologize first, what is PBO oc?
To expand on TJ's response...
PBO... Precision Boost Overdrive...Precision Boost Over-ride...Precision Boost Overclock. I'm not sure which it is, I've seen them all used sometimes by the same poster.
Ryzen CPU's call their core-boosting feature Precision Boost, Ryzen gen 2 expands on it boosing more frequently and longer and doing it with 2 cores/4 threads on SMT models. The 'X' CPU's will boost even higher and longer, they call that XFR boosting, and they can do that because AMD bin-sorted processor cores to put the best in the X CPU's...the best of the best going in 2700X.
Enabling PBO (override/overclock/overdrive) tells the CPU to ignore limits, as TJ says. But you can also relax other constraints in BIOS to tell it to also raise thermal limits, core current limits, etc. This allows those two cores to boost very high....usually much higher than you can achieve with a very aggressive all-core overclock. This all works much better on X CPU's because they received the best cores.
The advantage of that comes with games and simple multi-tasking apps that put a heavy processing load on only one or two cores. Even the best multi-threaded modern games usually have one thread that is the bottle neck, that thread gets to boost to 4.3-4.4Gig on a 2700X that's properly set up with a PBO overclock.
Another advantage is with core voltage offset adjustment you can actually lower core voltage (under volt) and it will keep the CPU cooler. That allows it to hold the overclock longer... or allows you to do this on stock cooling.