Google Search’s update to 2.8.7 includes a few more things than the change log originally said. The hotword has been changed to “OK Google,” and it’s now possible to use that hotword from search results, not just the standard search screen. To enable the new hotword features once you have version 2.8.7 of Google Search, head to Settings -> Voice -> Language -> Offline Speech Recognition, then download the language pack update. You might need to force close and reopen Search for it to start working.
This is brand new, and it’s still being determined whether it will work for all phones. For Android Police, it worked on their Nexus 7, Nexus 4, and Galaxy Nexus, but didn’t work on their Note 2. US English is the only language supported for “OK Google” at this point.
Source: Android Police