I use hemostats, that seems to work well enough. I would imagine small needle nosed pliers would work too. Either way you only want to grip the ring itself as not to scratch the post.
Before you ask, I have several nurses in the family including SWMBO. (Hence the hemostats).
Start with two fingers on the O-ring at one side of the post push around with a finger on either side and gap will form when it stretches. Just grab it and pull it off. It is pretty easy.