The ones that use the iPhone's camera can get heart rate, but I have not seen one that can get blood oxygen level.
I don't think blood oxygen level can be measured using the phone's camera. Also, I read an article lately regarding the low level of accuracy regarding heart rate measured using an iPhone camera. I'll try to find it and post it.
Depending on the spectrum of the phone camera's flash LED, you might be able to get away with just putting a diffraction grating in front of the camera. I recall, from reading a TI app note about pulse oximeters a while back, that these work by comparing the relative intensity of IR and a visible-red light transmitted through the finger (and that there was also some way to do it with reflected light). I wonder if there is a clever way to calibrate the grating without any extra hardware?
Heart rate variability requires more accurate measurement than a simple pulse measurement. I believe that some of the (older?) consumer heart rate monitors can't do HRV, so it'd be surprising if the iPhone camera hack could measure it reliably.
HRV is a pretty good measure of how stressed your body is, looks pretty useful for fitness training and illness/injury prevention.