Tap water has low pH.

[clm08k]

if you don't want to add chemicals...you could get planted tank substrate that will raise the pH or keep it at a neutral 7. Be sure it doesn't increase phosphates too much though.
Adding rocks could help raise it as well.

However, I don't particularly know much about the parameters for an Oscar.

 

[cichlidfish]

I use crushed coral to raise my ph. It works great. My tap water is pretty consistently 6.8 and the CC keeps it around 7.5.

 

[pops]

Oscar do fine in 6.2 to 7.8 unless your perams running out if that just not see an issue.