I am pretty sure that OK Google Now functionality was added to Lollipop 5.X for any phone that has Lollipop. Here is one of the Lollipop "new features":
Take Lollipop's always-listening voice command system out for a spin.
Provided your phone or tablet supports it, you can now give voice commands anytime -- even when your device's display is off. The option to activate the feature is a bit buried: Head into your system settings, tap "Language & input" and then "Voice input," then tap the gear icon next to "Enhanced Google services" and look for a line labeled "'Ok Google' Detection." Tap it, then tap the line labeled "Always on" and follow the prompts to train the system to recognize your voice. While you're in that menu, think about whether you want voice commands to work even when your phone is locked with a pattern, PIN, or password; if you do, tap the line labeled "When locked" before you exit out.
Once you're all done, just say "Okay, Google" and your phone will start listening. You can then ask it a question or give it all sorts of commands and have it work for you whether it's in your hands or not.