Yes. Cunnilingus, oral sex on a woman, is considered "possibly unsafe" without a latex or plastic barrier. If the woman has her period or a vaginal infection, it's considered "unsafe." "Possibly unsafe" activities have a smaller chance of transmitting HIV than behaviors in the "unsafe" category (which generally involve semen or blood coming into contact with mucous membranes in the rectum, vagina, or mouth). "Possibly unsafe" activities do, however, carry a risk of HIV (and STD) transmission.
Vaginal secretions can carry the virus and may be infectious under certain conditions. For example, if the secretions come into contact with open sores or small cuts in the mouth (such as those you may get from brushing or flossing your teeth) or the throat (such as when it's sore), there's a risk of viral transmission. If the woman receiving cunnilingus has her period or a vaginal infection, her secretions can hold a higher concentration of HIV, so there's a higher risk of transmission at these times.
You can also transmit other STDs during unprotected cunnilingus - like herpes, syphilis, hepatitus B, molluscum contagiosum, and yeast infections. And it is theoretically possible (but almost unheard-of) to transmit gonorrhea or chlamydia during unprotected oral sex.
To have safe cunnilingus, use plastic wrap (the regular household kind), or a dental dam, or a condom (unlubed, flavored if you want) cut up the side and held flat against the ENTIRE genital area, as a barrier. A little lube on the side next to her genitals will increase sensation for her; putting some jam or honey on the other side might make it even more fun for her partner. Be sure NOT to reverse the barrier. Only one side should come into contact with the genitals, and only the other side should come into contact with the partner's mouth. Also, don't use one barrier on more than one person, or transfer it from anus to vagina.
Yes. Fellatio (oral sex on a man) is considered an "unsafe" activity, meaning it involves a relatively high risk of HIV transmission. Fellatio is especially risky if the man ejaculates in his partner's mouth, but even the pre-come (drops of fluid that come out of the penis before he orgasms) can carry HIV. If the person giving the man head has any small tears or cuts in her/his mouth or throat (from brushing or flossing teeth, or during a sore throat, for example), HIV can be transmitted. There's also a risk that the man getting the blow job could contract HIV this way (albeit a small one).
To protect yourselves, use condoms during fellatio. Unlubricated ones are best for this - though some people find that putting a drop or two (but no more!) of water-based lube inside the tip of the condom makes it feel better for the man getting the blow job. Some people like to use flavored condoms for this.
You can also transmit other STDs during unprotected fellatio - like herpes, syphilis, hepatitus B, molluscum contagiosum, and yeast infections. It is also theoretically possible (but almost unheard-of) to transmit gonorrhea or chlamydia during unprotected oral sex.