Basically you can get any sexually transmitted infection or disease from oral sex, which you could with ordinary intercourse.
People often think that oral sex is safer, but in fact you can still get the HIV+ virus through oral sex and you are still at risk from Chlamydia, syphilis, gonorrhea and herpes if you have unprotected oral sex.
The infections listed above can all spread from the genital area to the mouth and then back from the mouth to the genital areas. HIV+ is caught by having small abrasions or lesions in the mouth and the receipt of bodily fluids which is part of oral sex (so to speak!).
It is however, very true that you cannot get pregnant from oral sex, but it is still an activity which can result in a STD, even man to man or woman to woman or within a heterosexual couple. Barrier methods are advised, such as using a condom or if the sex is with a woman, using a cut condom to protect direct contact between the mouth and genital area.

Oral sex can transmit most of the STDs,syphilis,Herpes,Chlamydia(LGV),Chancroid(Hemophilus ducrei),HIV and gonorrhea.However pharyngial gonorrhea is usually asymptomatic.
Best method of prevention is to avoid multiple partners,practise safe sex,and once you develop any ulcer(painful or painless)report to the doctor immediately which requires treatment of both the parties.Lesions of syphilis are readily curable during the initial stages,dreaded among all is but obvious HIV so protect yourself and protect others!