The Cyanogen team recently released the highly anticipated CyanogenMod 12S update for OnePlus One featuring Android Lollipop and the material design elements of visual, motion and interactive design.
Apart from the Lollipop sweet treatment, the update adds always-listening functionality which wakes up the OnePlus One when you say "Hey Snapdragon".
Cyanogen claims that the feature shows up on Setup Wizard on a fresh install or, can be found under "Language and input" settings, but users seem to have been experiencing issues accessing this.
Reddit user, TrueJournals, has found a workaround to activate the Voice Wakeup feature on OnePlus One. The method is simple and uses Voice Wakeup apk from a custom Mod from CyanogenMod 11S.
The only caveat is when the lock screen is enabled, the phone will respond to "Hey Snapdragon" but it will not get past the lock screen. However, if the lock screen is disabled, Voice Wakeup will work as you want it. Check out the following guide if you want to enable Voice Wakeup on OnePlus One.
How to enable Voice Wakeup 'Hey Snapdragon' on OnePlus One running CyanogenMod 12S
Step-1: Root OnePlus One running CynogenMod 12S
Step-2: Download VoiceWakeup-signed.apk (CM11S). Extract from the system folder
Step-3: Copy the apk file to the phone's storage or, sdcard
Step-4: Setup ADB and Fastboot on your PCX. Click here for a guide
Step-5: Enable USB debugging on OnePlus One. Connect it to the PC via USB cable. If it asks for USB debugging authorosation, click OK
Step-6: Open a command window on PC and execute the following commands in the given order
mount -oremount,rw /system
cp /sdcard/VoiceWakeup-signed.apk /system/app/VoiceWakeup/VoiceWakeup.apk
chmod 755 /system/app/VoiceWakeup/
chmod 644 /system/app/VoiceWakeup/VoiceWakeup.apk
Doing this mounts/system as writable so you can install APK, make the necessary directory structure, copies the file in, fixes some permission then remounts/system as read-only
Step-7: Finally, reboot your OnePlus One. The setup wizard will run automatically; if that does not happen, go to Settings>> Language & Input >> Voice wakeup
It is recommended that you Disable the Google Now "Hey Google" functionality as it might interfere with the "Hey Snapdragon".