Thanks also for reading other threads with questions similar to your own. Indeed oral sex can be considered safe sex, with low risk of all STIs and extremely low risk, perhaps zero, for some. The three main STIs for the penile partner after oral sex are gonorrhea, herpes due to HSV1, and nongonococcal urethritis (NGU). NGU from oral sex probably is often caused by entirely normal oral bacteria and likely is harmless, both for affected men and their sex partners. Gonorrhea or herpes can be a big deal, but the actual risk after any single exposure is very low. Hepatitis B and HIV fall into the near-zero-risk category.  

In general, we do not recommend any STI testing at all after any particular oral sex exposure, unless and until symptoms develop, typically in 2-5 days for gonorrhea or herpes, 1-2 weeks for NGU, and 2-4 weeks for syphilis. However, if you would like to be tested for reassurance, I would suggest waiting until 3-4 days have passed and then have a urine test for gonorrhea. (Chlamydia testing almost always is done when testing for gonorrhea, but there is little or no risk of chlamydia from oral sex.) You could also have a syphilis blood test in a few weeks. In absence of symptoms, I would not recommend testing for HIV (there has never been a proved case transmitted oral to penis) or herpes (without symptoms, the available tests are not sufficiently reliable). And also no need to test for hepatitis B virus infection; the risk by oral sex is too low. Also, almost all Thais are vaccinated against HBV, so there is little chance your sex worker contact is infected.

I hope these comments are helpful. Let me know if anything isn't clear.

HHH, MD

Just out of interest, how are you equating low risk in terms of probabilities to transmit? You see so many conservative statistics online that suggest any oral activity is high risk and it would be great to get the facts from someone who is eminently qualified. 

Why do some sources take the all or none risk viewpoint? Some are simply naive about the available data. Others may, as you imply yourself, have a political or social policy perspective, hoping to dissuade people from doing things that, in their fevered minds, are immoral or otherwise undesirable. Still others have a legalistic slant, fearing risk of legal action if someone becomes infected after hearing their advice. And so it goes. But on this forum, we stick with the available scientific data; and, where data are lacking (as for most STIs and most exposure types), reliance on known biological principles, anatomic aspects of sex, and so on. Without going into more detail, there are simply no data to suggest a significant risk of most STIs by oral sex; and I'll repeat that it is highly probable your Thai sex worker could not have had hepatitis B due to childhood immunization.

Gonorrhea is a "big deal" in that the risk is real (if fairly low) and easily treated, although sometimes causing dangerous outcomes before treatment. And yes, the chance of syphilis from such an exposure is extremely low; and if infected, you would probably develop an obvious chancre (syphilitic ulcer) of your penis. Without that happening, testing is optional and primarily just for reassurance.