It’s difficult to know the exact probability in these situation as there are so many factors to consider, for example does she even have an STI in the first place. What I can do is give you some general information on what STI could pass with the type of sex you had.
With the type of sex, you have described the receiving unprotected oral sex would be the one that could pass an STI. We don’t see HIV passing when receiving a blowjob, but STI like gonorrhea, chlamydia, herpes, syphilis and HPV could be passed if the person performing the oral sex had one of these in their mouth. I don’t think having the oil applied afterwards would change the chances of an STI passing.
Even if someone had one of these in their mouth it is never 100% that it would pass the first time you had sex. Many STI do not show symptoms so if you are concerned something could have passed I would recommend going for an STI test.
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