An optometrist who can state through documented experience that you know what the profession of optometry entails.
A professor with whom you have done personal work (such as assisted on a specific project or served as a TA or reader), or with whom you took a course.
An employer or extracurricular activity advisor who can comment about your maturity, diligence, and conscientiousness.
This is what OptomCas says. My interviews took the information that pertained to them and ran with it, there are no strict guidelines as far as I know but be sure to check the individual sites for the schools you are applying to.