That's a question with no easy answer. Oral sex isn't the riskiest activity. It's up to you to decide what your limits are and what's safe enough for you. Using a condom for oral sex is the safest. A little flavored lubricant or honey, or any food without oils, can spice things up.
Oral sex is safer without ejaculation.
But precum contains HIV, too. Be aware of the state of your mouth. Bleeding gums or cuts can increase the risk of infection. Don't brush your teeth prior to oral sex, or if you do be gentle as to not scratch your gums with the brush. But you can get hepatitis, gonorrhea or other sexually transmitted diseases.