An oxygen concentrator Christmas tree isn't a festive decoration that you put up in your home – it's used as an important little piece of hardware that helps to make sure the oxygen tubing stays in place on the concentrator.
The Christmas tree adapter gets its name from its shape and appearance. It looks like a Christmas tree, with it's conical shape and tiers that resemble the branches on a pine tree. It's also always or more often than not, a green color. It's an easy name because it describes how it looks in a way that most people can understand.