I have a small metal tube the same diameter as the o-ring that I drilled out to accomodate the piston stem. I can use it to push the o-ring into place with the piston already installed, stops the o-ring from trying to turn or pop out of place.
One of several tools I've built to make things easier. But you could also use a small dowel rod to push the o-ring in to place. I don't like using hex keys for it, because if the o-ring is stubborn the hex key can dent/cut it up a bit.