Negative Oxygen Consumption Rate
The diagram is not clear. They should write "change of oxygen consumption rate". 

The caption of the diagram makes it clearer what they are describing:

" In A, a time course for measurement of oxygen consumption rate (OCR) and extracellular acidification rate (ECAR) is shown under basal conditions, [...] ".


So basically they are measuring oxygen consumption rate under basal conditions and apply some chemicals and depict the change in oxygen consumption rate in a diagram, given in percentage.