It is possible, but most people I know don't work outside the school. All most everyone I know has a work-study job through the school though. At ICO there are two wages, optometry wage and non-optom. wage. 1st years make $9.80 or $6.80, 2nd years its $10.80 or $7.80....it goes up by a dollar from there. Basically the job breaks down into can you study while you are working or are you actually working the whole time.
The best thing about the on-campus jobs is that your boss (usually a prof.) understands the stresses you are under and helps work things around your tests and classes. Personally I think $10.80/hr for a job that is there when I want to work it is pretty great.
Last quarter I TA a lab, picture edited for a prof, and was the AV guy during lecture...I was making ~$700/month. That was a great help for bills! It's not like you could pay tuition with that, but if you are just looking for a little extra cash I would look for on-campus jobs first.