Your odds of picking up HIV with oral sex are rather low. There are precautions you could take to further reduce the risk (though it is rare for this to be done, and hardly anyone gets HIV just from oral sex anyway). But there's no way really to reduce the risk to absolute zero. Accidents happen.
On the other hand, there are significant odds of catching certain other STIs through oral sex. Some, such as gonorrhea and chlamydia would likely be blocked by the use of a condom. Others, particularly the skin-born types, like herpes and warts, could easily be shedding from outside the area a condom would cover. For those types of viruses, your only tool to avoid transmission really is examining the person you're hooking up with. Apparent lesions/warts should probably be avoided. Shedding can occur regardless, so having some knowledge of the person would help.
Also, keep in mind that exchange of bodily fluids through the blood, particularly open wounds, is going to drastically increase chances of transmission of HIV and hepatitis.