Unfortunately you can indeed catch an STI from giving oral sex to a female partner. Some are less easily transmissible but gonorrhea, herpes and HPV (the virus that can cause genital warts) are spread via genitals-to-mouth contact for example.

It's also not usually possible to see if your partner has any of these STIs. If a woman is going through a herpes outbreak then that will be fairly obvious and dude, you'd know not to go forth right?

To be safe, it's recommended to use dental dams (a flat bit of latex) for oral sex on women but if that sounds about as appealing as licking an icy pole covered in Glad Wrap, you'll need to assess the risk yourself. If your partner is someone you'd like to see regularly, then it's worth being tested (both of you) in the name of happy and safe shagging.