First and foremost and the most obvious is that you need a male and female. How you get a male and female is up to you but really the best way is to buy a group of 6 juveniles. they need to be in a large tank from the start. From this group it is highly likely that you will get a pair and if you obtain a pair they will spawn providing that you provide the right conditions. Frequent(weekly)water changes of 15-20 % are essential as is a varied diet. If you have a tank of 5-6 large Oscars and a pair form,then you will notice that they start to dominate one section of the tank keeping the others at bay. At this time you should either remove the pair to another tank or rehouse the others. The second option is the best. To encourage the pair to spawn you should feed them on live food continuously for a couple of weeks. plan a larger than normal water change and then stop feeding them for a couple of days. On the day of the water change, top up your tank with cooler water mimicking the change in the water temp that Oscars experience in the wild. Start to feed your Oscars more than you normally would. Try to feed them things like garden worms,crickets, meal worms or any insects you can find in the garden. This again mimics the conditions found in the wild when during the rainy season insects are washed into the river. this should encourage your Oscars to spawn. Water chemistry is not important as long as they are not extreme any ph between 6.5-7.5 will be fine. If you are able to collect some rainwater and use this for your water change.The hardness of the water is also unimportant. The water in my area is really hard as the area is quite chalky but i have never had a problem in breeding my Oscars.I have had several people who wished to breed Oscars exchange what they thought were a pair of Oscars for six juveniles because they could not get there pair to spawn. I have taken the large Oscars put them in a tank on their own and low and behold they have spawned a few weeks later. so that could be something you might try if your Oscars go through the motions but dont spawn. Set up another tank leave it for a few weeks to mature and then transfer the Oscars into the new tank and this may encourage the to spawn. (dont forget to move your filter to the new tank)If you require any other info please ask.
GOOD LUCK