Hello @naffie, I would like to add my welcome to Connect along with @merpreb and others. My brother-in-law has COPD and uses an oxygen concentrator. I'm have no medical background or knowledge but did find this layman's explaination that makes sense. The text is from the link below...

Quote from "What is the difference between a ventilator and an oxygen supply installed to medical patients, in simple language?"
-- https://www.quora.com/What-is-the-difference-between-a-ventilator-and-an-oxygen-supply-installed-to-medical-patients-in-simple-language

A “ventilator” (mechanical ventilator) is a machine that takes over the work of breathing for a patient with primary respiratory failure due to lung disease or secondary respiratory failure due to central nervous system disease or anesthesia / drug overdose. Ventilators are connected to the patient via an endotracheal tube (“breathing tube”) that is placed in the patient's trachea (“wind pipe”). Ventilators remove carbon dioxide and deliver oxygen to maintain physiologic homeostasis (normal blood pH).

Oxygen can be provided without endotracheal tube to those with adequate respiratory function (i.e., do not need a ventilator) but do need supplemental oxygen due to, most commonly, chronic lung disease (e.g., bronchitis, emphysema, acute asthma attack, cystic fibrosis, etc). Usual methods for administering oxygen are via nasal cannula or various masks.