Your ph has nothing to do with it, they don't add ph modifier to their water, so unless you have well water, or live far away from that particular store, your ph would be the same, but thats beside the point, the ph, unless its fluctuating is not going to kill your fish, you should contact the store manager and demand a refund for the oscar, as well as the test kit and ph modifier, try to remember the name of the idiot who sold you that stuff
EDIT: please be more careful in handleing your future fish, and in picking ones that will be compatible