Undervolting is never physically harmful. The worst that can happen is BSOD's from the voltage being too low, but that won't damage anything.
And OC'ing will always shorten the lifespan of the component that you're OC'ing somewhat, but a relatively modest OC with good temps usually won't shorten it extensively. That has more to do with the voltage that you need to get to the OC that you want, though. Higher voltage = shorter lifespan.