I don't know of any fish that require a specific KH value. You'll never see KH listed in water requirements for fish. You may be thinking of total hardness, which is GH. Even with that, no fish requires a specific GH value. Some do better in a range or will only breed in a range.
From what I've seen, oscars prefer a softer water. It seems to me that they'll probably do just fine with the water that you have. I wouldn't even bother lowering the pH. It'll lower on it's on as the nitrogen cycle acidifies the water. KH will actually drop between water changes, and pH with it.
You need to just be concerned with KH dropping to 0 and pH dropping drastically to < 6 as a result. Frequent partial water changes with what you have should be just fine, particularly if your tap is a bit soft. Go for stable values within an acceptable range rather than adding stuff aiming at attaining a specific value. Frequent partial water changes will accomplish that, assuming the tank is not overstocked or overfed.
If you can get and post your tap pH, GH, and KH, I could confirm the recommendation to just go with the tap as is. First choice is always to just use the tap as is, if possible, rather than trying to adjust it with additives. Most fish do very well in a wide range of values.
I have rift lake cichlids and buffer a bit just to avoid pH crashes. My values vary a bit from reading to reading and the fish are no worse for wear.
Most other posts I've seen here would confirm that reaching and maintaining a specific value for most fish just isn't necessary. Good nutrition, compatible tankmates, clean, well filtered water, and frequent partial water changes are more important to focus on.