The first time you start ChromeOS it asks which search engine you would like to use. You are then free to use any web app you want. The OS is open source so you can check for any tracking code. If you don't trust the build that comes with your chromebook, you can install your own chromiumos build on top. You can't do the same with Windows or OSX, you have to trust in Apple and Microsoft that they won't spy on you.

It's hard to get any more transparent than that.