The Oscar tanks are kept at 5 or below with lots of water changes. If it's good enough for Discus it's good enough for my Oscars, I always say. The other 17 tanks (including the Discus tank <g>) are kept below 10 easily with 1 major water change a week. We have to pick our battles, my battle is excellent Oscar water.
If it matters I use the API liquid test also.