Translating 9 - 13mV to \$\mathrm{O_2}\$ level
Gain = 5 for 0 to 100% range = 9 to 45mV (min offset).
Offset = 9 to 14 mV.

first you adjust for offset error using 100% N2

second you adjust using 100% O2 for calibration as defined in sensor manual

Using a 4 point linear curve fit may reduce gain error &lt;1%


Load : 10 kohm min.
You can choose any gain and max out range you want.  What do you prefer?
How do you intend to calibrate? s/w or h/w?