Currently, you can use either Ok, Google or Hey, Google to initiate a voice command with Google Home devices. The latter wake command has never worked on the mobile version of Google, until now that is.
Google is now rolling out the ability to activate Google Assistant by saying Hey, Google instead of the more cumbersome Okay, Google. While not a significant change, its one worth mentioning.
The new hotword was first discovered several days ago by XDA-Developers in an APK Teardown of one of the most recent updates to the Google App. Users who receive the update will receive a notification, prompting them to retrain their voices within the Google App.
Let us know if you have received this update and which phone it was on.