You will need to use both a red LED of the correct wavelength, and an IR LED of the correct wavelength, and pulse them alternately, to do the oxygen measurement. As Kleinstein indicated, you really ought to start working with that pulsed approach now. Pump all your transmit energy into a brief pulse, and detect at the right moment. This will improve your SNR a lot, and reduce things like the 100Hz/120Hz pulsing of most room lighting. When you get to measure the oxygen, analogue filtering of the signal will be a problem, unless you have two analogue chains, and switch between them as you switch between the LEDs. Its really better to sample the raw transimpedance amp signal with an ADC, and do all the processing with two chains of digital filtering. You are looking for a small signal with a huge DC offset, which requires some careful handling to keep the signal within range. The TI app note which Kleinstein linked to, and the earlier versions of that for older MSP430 parts, gives some information about the issues that need to be addressed, and one set of solutions.