chart of risks for various types of unprotected sex. Note that immune status and general health, presence/absence of openings in skin or mucous membranes (cut on lip, razor cut), etc.., etc.., etc.. all have an impact on risks as well. Note also that this chart is for the general population, and risks among a population that tests regularly (such as hobbyists and providers) may in fact be a bit lower, assuming detection, treatment and abstaining until the infection is cured is taken more seriously and rigorously followed.
Your best option for some high risk oral transmission (Hepatitis A and B) is vaccination.
A Herpes vaccine is in clinical trials and will soon be available. Note that there are two types of herpes virus (HSV-1 and HSV-2). As much as 70-80% of the general population has HSV-1 and up to 20% has HSV-2. Avoiding sex during an outbreak and taking preventative antiviral therapy reduces transmission risk immensely. Also be aware that herpes infections can be transferred to areas of the body other than the mouth and genitals; at one time it was common for dental workers to acquire "Herpetic Whitlow", an herpes infection of the hands/fingers. A condom will not protect against infections on areas outside of the penis/mouth. Note from this that herpes is simply an infectious disease that is transmitted in many ways - not just through sexual activity.
Condoms are most effective against orally transmitted bacterial infections such as syphilis and gonorrhea, chlamydia, etc.. as they cover the urethral opening. Most transmittable oropharyngeal gonococcal infections clear up relatively quickly on their own, without AB treatment. Transmittable oropharyngeal infections from other common bacterial STD's are nearly nonexistent. Note that infection risks from unprotected oral sex are already much, much lower than risks of transmission from unprotected penile/vaginal or anal sex.
There is no zero-risk scenario for sex of any kind with another person. However, the relative risks are lower with oral sex than with intercourse. Condoms are one way to further reduce risks. You simply have to decide what level of risk you are comfortable with.