Using a condom for oral sex greatly cuts down the risk of sexually transmitted infections, which can be passed in either direction during unprotected oral sex.
For safer oral sex on a man, put a condom on the man before you get started. Don't like the taste of condoms? Try using flavoured ones.
For safer oral sex on a woman you can use a latex square called a dental dam. You can get these from some chemists, online, from some sexual health projects or you could try at your local Sexual Health Service. They come in a range of flavours too. Alternatively, use a cut up condom. (Female condoms are better for this as they're a bit bigger.)
You should also use a cut up condom or a dam if you want to lick or kiss the anus.