Originally Posted by FlyerGoldII

See below (regarding Pearson airport) :

"OSS on a boarding pass is used when you arrive from the USA or international station, and are connecting to an international flight. When doing a connection of this nature, you must first see Canada customs officers and then a GTAA agent. Depending on if the boarding pass has OSS, the GTAA agent will direct you to a security check point, or to continue onwards to the gate area in international.



You will always go through security when connecting to a US or Canadian destination ..."