it really depends on how cold the water gets when you do get a power cut. Oscars are okay would water down to 21°C so if you are at risk from power cuts, I would advise you to keep your water on the warmer side.
How many hours does the power go off for? Apart from the temperature dropping, you've also got to think about your biological filter. The bacteria that lives in the filter needs oxygen to survive. They get their oxygen from water moving through the filtration system. Obviously if there is no electricity, your filter isn't going to work.
I couldn't possibly tell you how long Oscar would last in a tank with no electricity powering the heaters or filters, and I'm not about to carry out any experiments, sorry:lol:
As long as your water temperature doesn't drop too low, the Oscars will be perfectly okay. They may start breathing heavily because of oxygen depletion but that won't hurt them in the short-term. If the temperature was to go off for many hours, I would be extremely worried then.