If you want to watch Youtube in your web browser, you can use the vorpX desktop viewer (Windows 8+). Alternatively The Oculus software also has a desktop viewer built in, probably also Windows 8+.
If you want to watch movies downloaded to your PC, you can do that the same way, use MPC-HC or VLC with vorpX (supports 3D movies) or use one of several dedicated VR video players. You can probably even find some free ones in the Oculus Store.