Condoms cover the head of the penis, which means that germs can't enter your urethra. That's how things like gonorrhea, chlamydia, HIV, hep B, mycoplasma and trich infect you. (Not all of these are transmitted by oral sex - I'm just explaining how condoms work in general.)
Gonorrhea and chlamydia can be transmitted via oral sex, and if the head of your penis was covered, you're still protected.
Herpes, HPV, and syphilis are transmitted by direct skin to skin contact - mouth to genitals, genital to genital, and genital to anal. Condoms offer significant protection against these, but since they don't cover all the skin in the area, it's not full protection.
Even if the condom slips down during oral, as Guitar says, STDs are transmitted less often during oral, meaning the risk is lower. Syphilis and HPV occur less often in the mouth than on the genitals, so the chances of getting those from oral are significantly lower.
You can get genital herpes type 1 from receiving oral sex if you don't already have oral herpes type 1, but 67% of people under 50 already have that, so there's a good chance you do.
There's just not a lot of reason to worry here.
I'm glad you take protection seriously. Protection even during oral is a good idea. Oral sex, though, is very low risk for std's. I understand that the base of your penis was exposed due the condom pulling up a bit but it is still low risk. If you do not get any symptoms, and you probably won't, you don't even need to test. Oral sex is low risk WITHOUT a condom but add that barrier in? You really had very little chance of any std to transmit and lodge. I would not be worried.