Oxygen is a constituent of many of the most abundant molecules detected in exoplanetary atmospheres and a key ingredient for tracking how and where a planet formed. We detected from the ground of the neutral oxygen 777.4 nm triplet lines in the transmission spectrum of the ultra-hot Jupiter KELT-9b, the hottest known giant planet. This detection has been inspired by novel non-local thermodynamic equilibrium radiative transfer calculations, which also result in matching the data significantly better than models computed assuming local thermodynamic equilibrium. We will present the models and the oxygen detection, further describing the constraints these provide on the properties (planetary mass-loss rate and wind speed) of the planetary atmosphere.