he probably means the water your using for water changes, the pH will often shift quite a bit when left stagnant, due to the dissipation of chemicals like co2 in the water, i would just leave it, oscars will do fine in pH 6.5-7.5 and high and lower just leave it steady that is the most important, we have oscars here that thrive here (are non-native) in very bad conditions from very dirty canals with no water movement to brackish water