Android Police unearthed evidence attesting to the offline functionality of OK Google in the 4.8 version of the app. While going through the application's APK, the guys there identified strings of code which allow the application to make calls, send texts, play music, etc. even when not connected to the Internet. At the moment, if you try to use the Android assistant in offline mode, the only thing you will get is a message which says that the service "can’t reach Google at the moment."
Additionally, the investigation into the APK also revealed a few other interesting facts. For starters, the app will tap into the data Google collected about various networks to notify you when you are using an insecure Wi-Fi connection. Next, there was some evidence of a few new voice commands that could possibly be added in the future. These will allow the user to instruct OK Google to change the phone's volume or modify the brightness of the screen. Last but not least, the code dismantling also revealed an icon hinting at a new widget which will allow you to create OK Google reminders without having to talk to your phone. This would be great for those situations when you need to add a reminder, but can't use your voice because you're in a meeting or in class.
Just to clarify the situation, at the moment, all these are just speculations and none of these functions have been actually confirmed by Google. From my experience, Android Police (the website which revealed the information) is a reliable source of information, but it may be a while (if ever) until the IT giant actually makes these features function.
Source: Android Police