oksigeno

A name that is a mistake, and we all still use it. Lavoisier named the gas in the 1770s on a theory he was confident about: that this element was the thing which made acids acidic. So he built a word for it out of Greek oxys, sharp or sour, and -genes, producing. The acid-maker. Within a generation chemists established that the acid-making element is hydrogen, and the name was already fixed in French, German, Russian, English and everything downstream of them. It is one of the neatest examples in science of a label outliving the idea that produced it, and Esperanto inherited the error along with the word.

The seam is real, though it is Greek rather than Esperanto. You can see the same -gen- in hidrogeno, the water-maker, which really does make water, and in nitrogeno. And oksid- for an oxide shares the first half. None of that is cuttable in Esperanto: oksigeno is one root, and the pieces are visible only as history. That is the ordinary situation for the international scientific vocabulary, which Scienca Esperanto sets out in full, along with why the language borrows those terms wholesale rather than building them from its own roots.

Using it. Oksigena is the adjective, oksigeni is to oxygenate, senoksigena is oxygen-free. The everyday neighbours are aero, which is the mixture you actually breathe, spiri, which is what you do with it, and gaso, which is the state it is in.