The risks of giving cunnilingus are very low - in fact, the risks of receiving it are much higher (particularly of getting candida or bacterial vaginosis).
The two most cited studies are meta-analyses from the late 1990s.
1. Edwards S, Carne C. Oral Sex and the Transmission of Viral STIs. Sex Transm Infect. 1998 Feb;74(1):6-10.
2. Edwards S, Carne C. Oral Sex and the Transmission of Non-Viral STIs. Sex Transm Infect. 1998 Apr;74(2):95-100.
I've put a link to both below, and I'd urge anyone who is interested to read them. It is dry reading, but very factual. My attempt at a rough laymans summary below:
Giving oral sex to a man has been identified as a risk factor for:
HIV (both with and without ejaculation) - NB, low risk (see below)
HPV (genital warts)
Syphilis
Gonorrhoea
Chlamydia
Giving oral sex to a woman has been identified as a risk factor for:
HIV - NB, low risk (see below)
HPV
Gonorrhoea
Receiving oral sex as a man has been identified as a risk factor for:
HIV - NB, low risk (see below)
Herpes (very few reports)
Syphilis
Gonorrhea
Receiving oral sex as a woman has been identified as a risk factor for:
HIV - NB, low risk (see below)
Candida / Thrush (highest risk)
Bacterial vaginosis (also highest risk)
Herpes
HPV
Note that the viral report makes reference to case examples of HIV transmission from oral sex. These have been disputed pretty aggressively since and are generally believed to be artifacts (false positive observations of low-frequency events because of misreporting)