"OK Google" is a great tool. With Google's ever growing and ever knowing knowledge-base constantly expanding there is pretty much nothing Google can't tell you. With its OK feature, obtaining that information is even easier, as all you have to do is speak your question. That is, as long as OK Google can understand you. Half the time, you ask one question and will get a different answer. That said, it is a useful feature and one that seems to get used quite often.
Although the feature is now prevalent on nearly all android mobile devices, it did take a little longer to become integrated into other Chrome products. However, over the last year, both the Chrome browser and Chrome OS also received the hotword function, although it did come with a caveat. Up until now, OK Google had to be used when in a new tab or browser. For some reason, the function was not automatically responsive, like it is on mobile devices. However, that now looks to have changed. Francois Beaufort announced on his Google+ page today, that the OK Google hotword function is now permanently live and working on Chromebooks. As such the feature now should be working in all instances and whenever your Chromebook is on and unlocked.
If you want to give the new feature a spin and see what questions Google can (and cannot) answer, then you do need to make sure your Chromebook is set to a 'Dev channel'. As standard, all Chromebooks are set to a 'Stable channel' and when new features are introduced, they are normally introduced as experimental features. As such dev channel needs to be enabled to get the experimental features (like this one) to work. If you have not changed your channel yet, then you can change your channel via your Chromebook settings menu. After this and to get the experimental setting working, go back to the settings menu and scroll down to where it says "Enable OK Google To Start A Voice Search" (like in the image below) and click the tick box. Then just restart the Chromebook and the setting will be live. Let us know what you ask Google.