I have an alarm (the default one of course) for every morning. A few months ago, I finally hot the OK google feature from any screen but it says the alarm is automatically dismissed. To demo this, try to put an alarm set to 1 minute and watch a video, the alarm will show up in the notification menu but not sound and say something like alarm dismissed.
Lastly, I also noticed OK google from any screen interferes with the voice recorder saying something like "There's an app already using the microphone" when I disable googles feature, it works perfectly.