Unix-based and Unix-like operating systems require an administrative password before being able to modify anything outside of the current user's home directory. The reason why OS X doesn't need third party security software is due to its Unix foundation. This isn't a discussion about the security of open source software versus the security of closed source, proprietary software. Google Chrome is also not open source, it's open source counter part is Chromium. If you are referring to open source software being more susceptible to malware than that is not the case. Both OS X and Windows are closed source and a lot of software that is written for both operating systems are also closed source.
