Originally Posted by
YVR Cockroach
Did you use the airside connection path (i.e., don't go down to the main clearance area)? Unclear if you did. Seems the only difference as far as OSS/security goes is whether your inbound flight is eligible or not. Scan the BP for your inbound flight as well as the connecting flight (so keep both handy if there's two). If the inbound is from an OSS-eligible origin, you get to bypass security and go straight to immigration. If not, get sent to security in the same area. It is/can be a long walk to domestic gates in either case.
Canada doesn't recognize air security from Singapore, hence the rescreening. It has a mutual agreement with the EU, and unilaterally recognizes screening at UK and USA airports. But all other points of origin will require rescreening, even if the passenger has ITD baggage transfer and doesn't leave the airside area.
In my experience, there is usually an airport employee at the connections centre (after passing airside customs) who will examine your inbound and onward boarding passes to see if you qualify for OSS.
In my personal opinion (though I don't get a vote on the matter) Singapore, Japan and South Korea should be added to the list of origin countries where Canada recognizes their airport security. However, I don't know if the terms of Canada's agreement with the EU preclude Canada doing so.