Can I have oral sex while I have a hydrocele?

Yes. A hydrocele does not interfere with sexual function. The latter will not make it worse or better either.
Yes. Oral sex will not affect your ability to have intercourse or oral sex. A hydrocele is simply a water collection around the testicle. One can still perform normal daily activities despite having a hydrocele.
Okay, as long as... Yes, it's harmless to have oral sex on a man with hydrocele as long as being mentally and physically comfortable.