Which is actually as it should be, since “ok google” is a feature you ONLY find right now on Nexus 5. It’s not even there on Nexus 7 (both 2012 and 2013) running 4.4.2. What I mean with “Ok google” here is that you can trigger it from a regular screen that just have the google search widget.
Why is that?
Since Nexus 5 got a launcher (Google Experience Launcher) that’s exclusively for that phone right now, but you can of course download it if you really want it.
But! I WANT “ok google”!
Then you need to install GEL and set voice search language to US english, then it works. Just tried on my One with 4.4.2/sense 5.5 and it works JUST fine. It does NOT work with any other voice search language, not even UK English. When you first set it to US english the Google search bar will tell you to ‘say “ok google”‘, but that goes away after a while. It’s still in “ok google” mode as long as you have it in US English and having GEL installed. GEL is now renamed Google Now Launcher, and can be found on Play store if you have a GPE or Nexus device
Edit: since Google updated Google Now you have “ok google” capability when you’re in Google Now and are using UK or US English as voice search language.
Edit 2: with the recent update (July 2014) you can now have “ok google” from any screen. More about that HERE
The link to facebook shows a clip of “Ok Google” on my HTC One.