Most soaps are basic. There are a few that are acidic, namely the flashy and expensive (and complete waste of money) antibiotic soaps.
Rinsing thoroughly is the best bet. Adding some vinegar to the mix wouldn't hurt plus it's easy to rinse off. Any potential left over vinegar won't hurt the fish, unless one of them get's fish-stick paranoia

. But in the end lots of rinsing.
The point of soap is that it has one hydrophilic (water loving) side that can rinse off and one hydrophobic (water hating) end which clings to oils. I don't believe that the addition of salt will help very much, I can see it potentially stabilizing the charged (water loving) end which is probably a bad thing, but I haven't made a study of it.
So back to rinsing. Plus, if the rocks are porous, after you've rinsed manically you should let them sit in some fresh water overnight, dump out the water and repeat. It sounds like a lot, but better overkill than fishkill.
Oh, if you have "hard" water, buy a jug of that RO or distilled or spring water stuff from the grocery store. The expression "hard" comes from the difficulty at getting soap to foam up, and you want soap as keen as possible to enter the water.
As for oxyclean, I've never looked into what it's made of, but if it's potassium hydroxide (KOH for the chem geeks among us, I know you're out there!) then it's perfectly safe. Using simple cleaners like KOH, vinegar, and ammonia are all fine for aquariums, they rinse off easily and in low concentrations pose no threat to the inhabitants, ammonia of course will be dealt with by your bio-filter. As for KOH and vinegar, measure the pH vs. tap water (left on the counter overnight to equilibrate with the atmosphere) and you can easily see how much, if any, is left over.
knowy may be ready to argue the laurels of bleach, and while it does have its uses, it too is a base and won't help remove the soap. He is correct that a little bleach in a lot of water is a great disinfectant, however in doses higher that say 1part bleach to 2 parts water bleach will not evaporate. In fact, to be precise, bleach does not evaporate. Bleach is a low concentration of sodium hypochlorite and water. Sodium hypochlorite is a solid, white, crystal, just like every other chemical

. Some will react to form chlorine gas, and as long as there's ample water left around to allow this to happen, it will, but it's not a fast process, boiling will speed it, but again, only so long as there's still water left around. In short, do not let your bleached articles dry before a thorough rinse and dechlorination.