The app DroidCam allows to share your smartphone camera via network or USB. It has several drawbacks, though:

• For a USB connection the Developer Mode and USB Debugging have to be activated and maybe drivers need to be installed on your computer
• The Android app on the phone displays advertisements, but is free to download or has a paid version (4,89€ as time of writing)

Basically, DroidCam runs on your smartphone and opens a port you can connect to. It offers three modes:

1. IP-Stream over Wifi
2. Webinterface for webbrowsers over Wifi
3. USB interface via Android Debug Bridge (adb)

Choosing the USB interface requires to install some software on your computer, but allows you to use your smartphone like a USB webcam. That means you can pick it directly as input device in OBS and use it like any other camera.

Full installation instructions are found on the webpage of Droidcam: http://www.dev47apps.com/

1. Install the DroidCam app (paid version)
2. Enable the Developer Tools by hitting your “Settings” → “About Phone” menu 7 times and then go to “Settings” → “Developer Options” and turn on “USB Debugging”. Start the app. Further details are in the app's instructions.
3. Install and start the DroidCam tool on your computer.

Now your OBS should show you the new camera (You may need to close and re-open OBS).