The risk of HIV transmission during oral sex is very low.
Condoms provide protection not only during vaginal or anal sex but also during oral sex (mouth-to-penis). When placed on the penis before oral contact, an external condom reduces the risk of catching sexually transmitted infetions (STIs). As long as only the man’s pre-cum, but no sperm is taken into the mouth it is not necessary to use a condom to prevent an HIV infection.
To reduce the risk of catching an STI during oral contact between mouth and vulva (vagina), you can unroll a condom on two fingers and cut it open on one side to make a rectangle that is placed on the vulva. It is not necessary to use a condom to prevent an HIV infection as long as no blood is taken into the mouth.