Can I install a new OS onto my HTC One? and if yes, how?
If you are talking about installing a custom ROM for the HTC One (M7 or M8) you will have to unlock the bootloader just like any Android phone. Any tutorial on how to unlock the bootloader should help greatly.
This seems to be decent tutorial after watching it:
www.youtube.com/watch?v=OP4VU_nPd28