http://www.dph.sf.ca.us/sfcityclinic/stdbasics/stdchart.asp
Adding on what rightly said by HSci34, none of the infected population contracted HIV through receptive oral sex (on male), there are however very few cases where performers of oral sex on men got infection when ejaculation was involved. Having said that, several other STD's including Herpes (also not curable) can be transmitted via unprotected oral sex.
Reeciving oral sex by men, is a theoretical risk, having academic value only. When it comes to HIV transmission, nothing is absolute zero (no risk) or absolute 100% risk free (like thought about condoms), unless you prectice total abstinence.
It's an extremely rare occurence to get HIV from oral sex. It's not zero, but very very close to zero risk.