STDs can be transmitted by oral sex.

STDs can be transmitted by oral sex, and in some cases, you may not have any symptoms for years. It may take 3-6 months for some STDs to show up on tests. More than 98% of people would test positive for HIV within 3 months of being infected, but in rare cases, it could take up to 6 months for someone to test positive.

A woman gave me oral sex and we may have rubbed genitals, 3 days later no symptoms of an std am i safe?
If your genitals touched she could give you warts, herpes among other things. Your penis does not have to be all of the way in her vagina in order to transmit diseases like those. And no, you're not in the clear after three days. Herpes or Warts can lay dormant for years without showing symptoms. I know, personally. Best way is just to get checked at your doc so you know for sure.

If you have this sort of worry I can not in a million Years begin to understand why you would go and mess around with someone especially someone whom you feel may not be well all that healthy.
Why would you if you thought she was not healthy engage in any type of sexual foreplay or activities?
USE A CONDOM!!!!!!!!Even for oral. It may feel different but It beats the heck out of ever going through worrying.
As said you may still have been infected with certain things so I would go get a check up and while there get a handful of condoms The local health departments have baskets of free ones available Make use of them.
Any woman who just runs around giving oral sex to a guy without a condom also should be rethinking what they are doing too.