On OSX what is the opposite command to "say", i.e. what command is there for speech recognition? I'd like to be able to enter a command that registers a speakable word that OS/X can listen for all the time. When the user speaks that word, it should launch an app. Similar to the "OK google" idea.

But is this possible these days? Last I heard of people doing this was in 2010.
If you go to System preferences -- Accessibility-- Speakable items
In settings turn on speakable items, and you can provide a keyword. After saying the keyword you can say commands.