Welcome to the forum. Thanks for a straightforward, brief question.
A similar question came up just 4 days ago; please read the discussion:
http://www.medhelp.org/posts/HIV-Prevention/Need-Education-Regarding-Oral-Sex-HIV-STD-Infection-Chances/show/1721189
In general, fellatio (penile-oral penetration) is quite safe, especially for the insertive (penile) partner -- not completely free of STD risk, but much lower chance of infection than for vaginal or anal sex, with low risk for some STDs and zero for other. There are no good numerical data, but I would guess that those listed below as "low risk" probably have well under 1 chance in 1,000 of transmission even if the partner has an oral infection. And since most potential partners in fact do not have active oral STD infections, the true risks are even lower.
The infections with low risk are gonorrhea, herpes due to HSV-1, and nongonoccal urethritis (probably often due to harmless oral bacteria, not true STD germs). Still lower risks exist for syphilis. There is exceedingly low risk (probably zero or close to it) for chlamydia, HIV, and trichomonas. There are no data on HPV, but certainly it's low risk.
For these reasons, we don't recommend STD/HIV testing after receiving oral sex, unless there are symptoms that suggest infection occurred.
I hope this helps. Best wishes-- HHH, MD