As you see from this nice video, changing the O-ring on the male threaded end of the hose (first stage) is very easy.
But, as said, this is static and usually has never problems...
Instead changing the O-ring at the second-stage end of the hose is much more tricky, and requires to have proper tools.
But this is a dynamic O-ring, so it must be mounted perfectly clean and properly lubricated. And, being dynamic, it is much more prone to failure than the first one.
In conclusion, in my "save a dive" bag I carry a spare hose, not the O-rings. If the O-ring at the second stage side of an hose fails, on the boat I simply change the hose, and I will take care of replacing the O-ring where back on the ground.
Fiddling with that tricky O-ring replacement on a boat rolling on rough sea can be really nauseating, and the risk to loose your tiny O-ring is large.
Finally, I had more often a hose failure (due to mishandling of the tank with the reg mounted) than an O-ring failure.
