The time I transited YYZ before the pictures, OSS was not in place. Assuming there is not another OSS entrance--If you go down the stairs, you would go see an agent and then go down towards baggage claim. My guess is that the FT'er who posted that you should go down the stairs is a NEXUS user. There are no NEXUS machines on the upper level, so it may be faster for someone to exit security and then go back to airside using the NEXUS line on the third floor...