There is still no Nexus in the YYZ OSS connection area.
You can obviously still go downstairs and use Nexus and then go back upstairs and re-clear security in YYZ if you want. However my OSS experience lately has been very fast customs & immigration upstairs and buses seeming to not be waiting around forever downstairs and I am pretty sure you're faster most of the time going that way instead. Obviously when they put Nexus machines in the OSS area and replace the bus with a secure corridor it won't even be close (think YUL), but even without a Nexus machine I can usually make it from arriving plane to domestic gate in less than 30 mins.