Not changing the water at all could be risky even with tough fish like Oscars.
If the pH is high and there is a lot of bacteria at work then you may end up with higher concentrations of amonia in its more toxic form. Lower pH tends to push the amonia concentration equilibrium towards higher concentrations of the less toxic form of amonia.
You should probably consider investing in pH, amonia, nitrite and nitrate test kits.
Without knowing more, I would sugest cutting back on the feeding for a while as well as increasing aeration and possibly lowering the temperature slightly. Doing this will increase the amount of available oxygen for the nitrifying bacteria to get established. Water dissolves more oxygen at lower temperatures and your fish metabolism will slow down, producing less amonia.
Once your amonia level begins to drop you need to concerned over rising nitrite levels. Nitrite is highly toxic to fish but once levels begin to rise, so will the population of bugs that eat it producing less toxic nitrate. Nitrate is usually metabolised by aquatic plants or reduced in concentration by water changes.
Often in new tanks (depending on your water source) changing water at a rate frequently enough to keep amonia and nitrite below dangerous levels, can prevent the establishment of a diverse healthy bacterial community by introducing toxins such as chlorine and chloramines that have been added for just that purpose by your water supply authority.
Continue with frequent small water changes but use only aged water. A fast way of aging water is to use a water cooler type water bottle (18 litre I think) filled with cold tap water. Drop a submersible 100 Watt or better water heater into the bottle set at the highest temperature along with an airstone for circulation. By heating the water you drive out virtually all of the dissolved gases including ozone and chlorine that may be introduced by your water supply authority to kill bacteria. The aeration will ensure proper circulation and uniform heating of the water. Keep the water warm and circulating for 12 hours and then unplug the heater and continue to aerate for the next 24 hours as the water cools and dissolves enough oxygen prior to being introduced into your tank.
Letting tap water sit in a similar sized bottle in most cases will age out in a 3 day period without having to be aerated and heated. Just make sure the bottle is clean and there are no small air bubbles adhering to the sides of the bottle. Using aged water will not kill your establishing bugs, reduce the amonia content below fish stress levels and provide micronutrients for the bacteria.
You may consider tossing a sponge filter into your tank to help accelarate the cycling process.
Hope this helps...
