We have no further customizations of the axis currently available in COPASI. So your options are to export the data, and plot them in another environment.
The other alternative is to post process the images saved. For example when you save the file as SVG and replacing the unit 'mol' by 'OD' in that file.
Finally the scripting interfaces like CoRC and basico would help here, if you run the simulation there, you can easily modify the generated plots.
best
Frank