Yes, you can activate a Google search by saying "OK Google." But only if you first navigate to google.com. Then, you can use the hotword.
Of course, it is faster to not navigate to google.com and just type your query in the Chrome omni search box. But I guess if your hands are messy, and if you are on Google.com already, saying OK Google is nice.
Here is Google's post:
Forum discussion at Google+.