After a recent update of the Google Search app (version 3.5 and above), it became possible to enable "Ok Google" from any screen on Android 4.3 and above. Follow these steps:
- Open Google Search on your phone
- Go to Settings by either pressing the hardware Menu button if your device has it, or scrolling to the bottom of all cards and tapping three dots, and select "Settings."
- Navigate to Voice -> "Ok Google" Detection, and enable the
From Any screen option.
- This will first train the device with your voice, and we are done
Note that this does not work on all Android devices. Some Motorola handsets like the Moto X have their own voice recognition software that supersedes Google's, and you will see a message
This option is not supported on this device.
Also, currently only the English language is supported for this feature, although Google will probably enable it for more locales in the future.