What anti virus software is available for Mac OS?

You don't need any.

You have heard wrongly. There are some trojans and Malware that affect macs but you need only know how to get rid of them if you find one. This is a good program.

https://www.malwarebytes.org/

There are many forms of ‘Malware’ that can affect a computer system, of which ‘a virus’ is but one type, ‘trojans’ another. Using the strict definition of a computer virus, no viruses that can attack OS X have so far been detected 'in the wild', i.e. in anything other than laboratory conditions. The same is not true of other forms of malware, such as Trojans. Whilst it is a fairly safe bet that your Mac will NOT be infected by a virus, it may have other security-related problem, but more likely a technical problem unrelated to any malware threat.

You may find this User Tip on Viruses, Trojan Detection and Removal, as well as general Internet Security and Privacy, useful: The User Tip seeks to offer guidance on the main security threats and how to avoid them.

https://discussions.apple.com/docs/DOC-2435

More useful information can also be found here:

http://www.thesafemac.com/mmg/

https://www.malwarebytes.org/

These articles from MacWorld are also useful:

http://www.macworld.co.uk/feature/mac-software/mac-antivirus-internet-security-s oftware-malware-review-3523842/

http://www.macworld.co.uk/how-to/mac-software/are-macs-safe-viruses-hackers-3454 926/

If you feel you must have anti-virus protection, try this one. It doesn’t affect computer operations as much as many do.

ClamXav

The best anti-virus/anti-malware tool for OS X is common sense about sites that you visit, and the advertisements that you may be enticed to click, or even just accidentally pass your mouse over.