You cannot get pregnant from having oral sex, even if you swallow sperm. However, you can catch STIs such as herpes and chlamydia.
Use a condom on the penis, and a dam (very soft, thin plastic square) over the female genitals to protect against getting or passing on STIs through oral sex.
Source: Sex myth buster @ NHS Choices
image by AdinaVoicu under CC0 license
