Oral Sex? Sure you can catch things! ANYTIME there is an exchange of bodily fluids.
Here ya go:
The usual suspects: gonorrhea, chlamydia, syphilis, herpes simplex virus I and II (oral and genital), and HIV....yeah, YOU CAN ACQUIRE HIV FROM ORAL SEX!!!!! It happens. There is substantial evidence that HIV is NOT transmitted saliva-to-saliva....but what if you have an open sore and the other person has a cut? Anytime there is a breach of barrier protection (i.e. skin, condom, etc) there is vulnerability and a risk.
The ones you don't always think about: Hepatits A and B, HPV (human papilloma virus) (multiple types...only 4 of which are covered by Gardasil...guys are recommended to get this vaccine since they are the #1 carriers of HPV)
HPV in guys can cause oral warts, anal warts, genital warts, penile cancer, anal cancer, just to name a few.
WEARING A CONDOM DOES NOT PROTECT YOU FROM HPV!!! It is passed on via skin to skin contact...period. So if your partner is wearing a condom and carries HPV around his pubic area, and your ass/lips/whatever touches in that area, you have a chance of contracting HPV.
Also, HPV is very prevalent! About 60-80% of all adults over age 30 likely are carriers. This does NOT mean they will develop symptoms. Since there are over 30 strains of HPV and only 2 strains (that we know) cause cancer and 2 strains (that we know) cause warts, there is a possibility that the HPV type is non-damaging.
Not trying to scare anyone, just trying to be informative. Know your body and be as safe as possible.
source: tons of shit I have to know forward and backward during med school
Disclaimer: None of what I say can or should be taken as medical advice. Please discuss issues such as these with your physician.