As per the Ministry of Education website:

The OEN is a student identification number that will be assigned by the Ministry of Education to elementary and secondary students across the province. The number, which is unique to every student, will be used as the key identifier on a student’s school records, and will follow the student through his or her elementary and secondary education. The OEN is nine digits long (eight digits, plus a check digit), randomly assigned, and tied to stable information about the student (name, gender, date of birth).

For more information please visit: http://www.edu.gov.on.ca/eng/document/brochure/oen/index.html

A student’s OEN number will be found on their report card.