Not quite as simple as that. 100% O2 for someone who is healthy won't depress respiratory drive as respiration rate/depth is driven by the level of CO2 in the blood. This has to do with the oxygen-haemoglobin dissociation curve. O2 at quite low concentrations in the inspired air still binds to Hb with such affinity that you will still float around 80% saturation. A partial pressure of oxygen of 50 mmHg will still give you O2 sats of ~80%, so you need to be pretty hypoxic before your body responds by increasing respiration. However, a slightly increased level of blood CO2 will greatly increase your respiratory drive (sensed by various neural structures in the body) as increased CO2 lowers the pH of your blood and can spell bad news if it isn't dealt with (this is irrespective of your O2 saturation).
O2 only depresses respiratory drive in cases where people have a chronic high level of CO2 floating around in the blood (COPD is one such condition). This attenuates the bodies response to CO2 and their respiration is driven by their levels of O2 (which is a much less sensitive system). Giving someone like this O2 can depress their respiratory drive, and in a worst case scenario stop them from breathing.
So yes, oxygen can be a respiratory depressant, but only in people who rely on hypoxic respiratory drive, not hypercapnic respiratory drive.
JD