Just wanted to share my experience as this if the first time I've used this (Service?).

DTW-YYZ-YWG all on AC. YYZ-YWG pass has ITD OSS.

I wasn't sure if I would be able to use nexus while doing this as there seems to be conflicting information, but I can confirm it was no problem.

Normal Immigration goes down the great big circular opening staircase. Right next to that are the large D and F connection signs. They are large, and very hard to miss. They are just to the right of the staircase.

There was an agent checking boarding passes - I'm not sure if they were checking specifically for OSS or not, but they waved me through and then it was down a short hallway to the actuall OSS setup. There is (as of the time of this post) 3 NEXUS machines, clearly marked and labeled, and about 20+ regular machines. After talking briefly to the officer, I was given my printed checkmarked paper back and then immediately handed it to another officer, and then it was another decent walk that puts you right next past domestic security (right next to the elevator to the MLL and the ground floor).

A very seamless process, the only reason I didn't get to the MLL faster is because we parked at the tip of F99, it's a serious haul!

But hope this helps clear things up for people. You most certainly can use both NEXUS and OSS.