Oxypowder and Colosan are the two top oxygen-based cleansers. They work on a very interesting chemical principle - mostly with magnesium and sometimes lemon juice or acidic acid of some kind. The combination allows for a constant release of oxygen throughout the intestines. Oxygen has a great affect - it liquifies impacted fecal matter and kills pathogenic anaerobic bacteria.
Based on what I've read - both product info and testimonials and CureZone users, it's best to start out with these types of cleansers, especially if you've had long-term constipation or candida issues. To be really thorough and reduce die-off symptoms, use the cleanser in conjunction with the occasional colonic/enema.