If you don't already have the Fitbit App, you can easily download and install from Google Play.
This is a good example if you need help in setting up the App. Also make sure to enable Bluetooth to allow the App to detect the Fitbit Charge HR and sync with the OnePlus 3. During setup, the App will walk you through some useful tips and information and also let you customize the fitness band.
Once setup has completed, the App will be able to sync automatically with the Fitbit Charge HR. Use the intuitive features on the App to make your workouts more exciting with the fitness tracker.