Charles can automatically configure the Mac OS X system proxy settings so that Safari and other Mac OS X applications automatically start using Charles.

When you first install Charles you will be prompted to grant permissions to Charles to autoconfigure the proxy settings. After that, Charles will configure and then reconfigure the Mac OS X proxy settings whenever Charles is started or quit.

Charles proxy configuration behaviour can be changed in Charles in the Proxy Menu, Proxy Settings dialog.

The Mac OS X proxy settings are configured in the Network panel in the System Preferences if you want to look at them yourself.