While it is possible to contract HIV through oral sex, it is statistically very unlikely. joelque is right to point out, per the CDC, that it can be hard to know when someone contracted HIV orally as people who engage in oral sex are usually engaging in other kinds of sex as well. That said, it's demonstrably easier to contract HIV through vaginal or anal sex than it is through oral sex.
To really understand the risks, you need to understand how HIV is transmitted, and not whether a certain type of sex is "risky" or not. The truth is, you can theoretically contract HIV through all sorts of sexual activities. For example, masturbating with another guy's cum as lube is risky if you have a sore or cut on your dick. If.
To contract HIV, you need to have a gateway into the body in the presence of a fluid that contains HIV. Fluid + HIV + gateway. Without all three of those, there's no chance of transmission.
If you have sores, cuts, or abrasions in your mouth (from illness, biting your lip, gingivitis, etc), then there's a way for HIV to get into you. Otherwise, human mouths are very inhospitable places for HIV.
So yes, you can catch HIV orally, but most HIV/AIDS agencies I know of consider it a very low risk.
Other STIs, such as syphilis and herpes, are more easily communicated, and are readily transmitted orally.