The assumption with the "O" ring is that you have a solid glass transom, or one at least well undercut, filled with epoxy in the way of the drain tube, and then re-bored to fit the tube.
A good other solution is to use sealant or epoxy to set the tubing (again after under cutting the core of the transom, if it is cored with any material (foam or wood). Two truck lug nuts will also work as flaring tools. You do have to cut the brass tube the right length.
I happen to feel that perhaps a better solution is to use a brass threaded fitting--getting away from the rubber plug. That is what I use on the boats I keep in the water.