I was thinking the same...other then that ONR will clean the black plastics I dont really see ONR doing much else. Its not a dressing.
Now Im not just saying this without trying it. I did use ONR today on some black plastics. I used it neat on a mf applicator and "dressed" the plastics and other then that it just cleaned it I diddnt see it do anything much to plastics...no blackening, no difference whatsoever. It looked stock as I expected it to be in the first place.
Opti seal on the other hand can be used as a protectant. That leaves behind a protected slick antistatic surface that actually beads water quite nicely.