I went through the OSS area in YYZ yesterday, while doing US-YYZ-YOW flights.
There seems to be better signage now with a 'Connections' sign there. As other have mentioned, it is very close to the area where you proceed down the escalators to the Immigration Hall.
A couple of notes:
1) I did not fill out a declaration form as I have Nexus. I saw there was no line up at the OSS area, so I went in. I then had to fill out the declaration form because you must use a Nexus machine to be exempt from filling it it. There are no Nexus machines at OSS...though the Officer did say that they new regular machines will be operational soon.
2) The bus is a pain- they wait until it is pretty full before departing- I think I waited 15 minutes. I guess it could be longer or shorter depending on the number of people there.
Considering I have gotten from door to door (though Immigration/customs and back though security in as little as 15 minutes, the OSS option seems very slow to me. I may not use it again.