“OK Google” is a pretty neat tool. It lets you search for virtually anything — as long as you’re on your homescreen. A recent update to Google Search unlocked that limitation and allowed users to enable OK Google everywhere, even on the lock screen!
The update requires Android 4.3+ (or 4.4+, depending on who you ask), it also needs the latest version of the Google Search app and the latest update to Play Services. Head out to the Play Store and make sure you’ve got all the latest updates installed before you continue.
Next, from your homescreen, simply say “OK Google” to bring up the voice search window, then say “OK Google Everywhere”. Some sources say you need to open a link (any link) on this screen, so pick one, and open it up. You might not need to do this, but sources say this “unlocks” the settings you’ll need to adjust in the next section. (Yours may already have been unlocked, but it won’t hurt to do this step anyway.)
Next, go to Settings, Language and input, then Voice Search. From there tap “OK Google” detection, and check the boxes next to where you want to be able to search: from the Google Search App, from “any screen”, and even from the lock screen. You’ll need to run through a quick training to teach it how you say “OK Google”, but don’t worry, if it doesn’t seem to recognize you later on, head back and tap on “Retain voice model” to run through the setup again.
That’s it! You should now be able to search using “OK Google” from virtually any screen on your phone.