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, it’s 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.