Oral sex takes various rulings depending on the situation:
a) if oral sex entails swallowing or likely swallowing of filthy substances–such as pre-sexual discharge–then it is prohibitively disliked and sinful;
b) if there is fear of this, and this fear is at the level of likelihood, then it would also be impermissible and sinful;
c) if the fear is not at the level of likelihood–such as when all precautions are taken–then it would be disliked and improper;
d) when there is no fear of swallowing any filth, even accidentally, then it would be other than the best thing to do (khilaf al-awla).
And Allah alone gives success.