Oral sex means using your mouth to contact someone’s genitals or bodily fluids (like cum). In general, it’s safer than fucking because your mouth is naturally good at stopping a lot of viruses and bacteria. It can still pose a risk if you have any cuts, abrasions, burns or irritation in your mouth or teeth.
Condoms can still be used for oral sex, as can a dental dam (basically a sheet of latex or rubber which you can put over a hole like an anus). Like the advice for condoms, these only work if they are used correctly, so you should play around with them to find out what works for you and your partner.
You shouldn’t engage in oral sex with your mouth if you have any cuts, or have brushed your teeth in the last hour or so (this can create small cuts in your gums). It’s also recommended that you don’t use mouthwash for a while before oral sex, as this can also irritate your mouth.