For file assocication, go to Windows settings > Apps > Default apps page > Select "Set default by apps" or other options comprehensively to try the configuration.
If the player doesn't show up on Windows for the file extension, ask your PC manufacturer for assitance to reinstall the preinstalled OEM version software (Media Player 14).
For the synchronization, did you enable any feature to load files from your OneDrive in Media Player 14?
OneDrive's synchronization is controlled by OneDrive app on Windows but not the software player.
If any programs access the files from the OneDrive's folder, it is expected the cloud stroage's synchronization might start accordingly. It has nothing to do with player feature.
Either adjust the synchronization setting in your OneDrive app, or remove the OneDrive or media folder to show up in the Media Player settings. You can also uninstall OneDrive apps on Windows if you didn't use it at all.