Heavy-oxygen water production
As far as I know, while fractional distillation would be a suitable technique to obtain O-18 water (yet, in very low yelds and in a way that is hard to control), the most used method nowadays is the membrane distillation technique: basically, a semipermeable and hydrophobic membrane is employed during the distillation (tipically made in PTFE), and the isotopes are separated by their ability to permeate through the membrane: lighter isotopes (O-16 water) tend to flow through the membrane, while heavier isotopes (O-18 water) are trapped in the apparatus and recovered.

You can find many detailed descriptions online, for instance:

http://www.ichtj.waw.pl/ichtj/library/Production%20of%20stable%20isotopes%20by%20membrane%20method.pdf

https://www.google.com/patents/US5057225