Well its not that difficult, you just need to ask them. I’m pretty sure that as your partner they trust you and understand and appreciate your interests. So, there’s no need to be afraid or anything just calm down and ask your partner if they would be ok with giving/receiving oral sex. If they disagree, you should also on your part accept their decision.
Just tell him/her that this is what you are looking for? Also give proper reasons why you are looking for this, even if it turns you on you can tell him that and if he says no then it is up to what you think is best for you?