Dallas,
Thanks for the correction :)
What you are planning would not work because the main STDs orally are Syphillis, Herpes. The only possible STD treated would be gonnorrhea. But that depends on the antibiotic you planned to use. Random use of antibiotics have led us now to see more and more resistent strains of bacteria. Why don`t you use condoms and vaginal condoms ? Eggplantz has several good comments. I just want to correct one thing. People don`t get resistent to antibiotics. It is the infectious bacteria that does.
In that case your chances of catching anything are slim, furthermore taking antiobiotics for absolutely no reason can increase your resistance to antibiotics. So when/if you REALLY need to take them one day they might not work.
I will not be receiving unprotected oral sex only giving it. I would like to take the antibiotics a few days after the encounter just in case I contract something.
The antibiotics will not do anything to protect you.
If you are receiving oral sex there is a chance of contracting HSV-1 he or she has it.