Chlamydia, in theory, can be transmitted via oral sex, but no expert has ever seen it happen.
Gonorrhea definitely can be. Using a condom protects against that, though, because the bacteria can't enter your urethra.
If you perform oral sex on a penis, you can get gonorrhea in your throat.
Receiving unprotected oral sex also puts you at risk for syphilis, NGU and genital hsv1, if you don't already have oral hsv1 (think cold sores, though you can have oral hsv1 without getting cold sores - most never get symptoms). Condoms offer full protection for gonorrhea and NGU (an infection in the urethra), and significant, but not total, protection against ghsv1 and syphilis.
Giving oral sex on a penis puts you at risk for oral gonorrhea and syphilis. Using a condom would protect you from oral gonorrhea, and offer significant protection against syphilis, but not total.