Cold crash suck back options

Maybe this is a dumb question, but is the amount of O2 being sucked in that much to be of significant issue? My thought is that since there's a "layer" of CO2 on the surface of the beer, any air getting sucked in will be above the CO2.

Or, is the real issue any air-borne yeast/bacteria/micro-bugs that might get sucked in.

FWIW, I cold crash in the keg while carbonating.

 

The new O2 will spread out to evenly fill the headspace (diffusion), including the part with the "blanket" (which is also busy diffusing throughout the space). Then it will dissolve into the beer until it reaches equilibrium (Henry's Law).

 

I use the plastic one piece " p-trap " style air lock and never had it suck in the star san solution and if it did take a little in, I've never noticed it.