Ps: Green LEDs apparently can be used for reflective oximetry, when paired with infrared. I suspect accuracy is not yet sufficient, or measurements not reliable enough yet, so Apple did not include oximetry in the initial release—but one can hope for signal processing advancements to allow for it in the future.
Hopefully they will enable O2 measurement in a future software update. The Apple Watch does have IR diodes in addition to the green ones. From <https://support.apple.com/en-us/HT204666>:
How Apple Watch measures your heart rate.
The heart rate sensor in Apple Watch uses what is known as photoplethysmography. This technology, while difficult to pronounce, is based on a very simple fact: Blood is red because it reflects red light and absorbs green light. Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment.
The heart rate sensor can also use infrared light. This mode is what Apple Watch uses when it measures your heart rate every 10 minutes. However, if the infrared system isn’t providing an adequate reading, Apple Watch switches to the green LEDs. In addition, the heart rate sensor is designed to compensate for low signal levels by increasing both LED brightness and sampling rate.