I keep the PH level at 7.0, & when I do a water change, I always add freshwater aquarium salt, 1 teaspoon of salt to 5 (US)gallons of water. Will this bother the Oscars? I haven't noticed any difficulty in their behavior, but I don't want to do anything that could hurt them in the future.

I personally think that it is not necessarily to use salt in an aquarium on a long-term basis. salt can be used to reduce the risk of nitrite poisoning, and if the fish has opened wounds, or fungal infections etc, in circumstances like this, it might be better to actually give to fish a salt bath, rather than putting salt in the aquarium. however, using salt on a long-term basis could actually have a negative effect, rather than actually keeping your fish healthy, which is what most people think. If I was you, I would stop using salt, it really is not necessary. Your Oscar is going to be no healthier with you adding salt.

and the other thing you've got to remember is salt can only be removed by the water changes

For what its worth I agree with OFL. I have never used salt in my Oscar tanks. I think its unnecessary unless treating a fungal infection and even then I am inclined to use API PimaFix.

I assume you use dechlorinator when adding fresh water during a water change

Lots of people keep oscar without adding salt so i would agree that is isnt necessary