I am looking for an optode to measure oxygen concentrations in seawater. I would want to maximise the trade-off between cost and quality. I have used fiber-optic and RDO based equipment before, but this technology is rapidly improving.
If you are doing your measurement in the sea, you may look at the optode from aanderra or the rinko from jfe alec (faster time response if you are doing profiles in the water column).
If you want to measure saturation you may want it coupled to a CTD in order to have salinity (you can check then dataloggers from RBR or Seabird).
What kind of accuracy, resolution, response time do you need ?