Before you go down the RO route, I would recommend trying Seachem Safe and a bag of Seachem Purigen to go in the filter, it will really help with crappy tap water, and if your water is anything like mine, there will be a lot more than just nitrates in it!
You may already know what both of these are, but if you don't, here's a quick explanation of why they are good for bad tap water.
Purigen will help you by removing Ammonia as it is produced, stopping it from ever turning into Nitrates, which gives your filter a chance to concentrate on sorting out the bad tap water. It also removes tannins, odours and basically acts like carbon. It's a great water polisher. Seachem claim it wont leach anything back into the water once it's full, and it visibly changes colour when it needs recharging. You recharge it with a weak bleach solution, then soak it in Safe to remove it. It's very very cheap for how long it lasts, and you don't need much of it at all, so it doesn't take up much space in the filter! If you do get it, I recommend getting twice as much as you need, so you can have one recharged and waiting to go in. It takes a day or two to recharge so it's much easier to have two lots of it.
Seachem Safe is just a powdered version of Seachem Prime, but it works out crazy cheap compared to Prime. It wont remove the nitrates, but it will detoxify them so that it wont upset your fish, and it still allows them to be accessible to your filter bacteria. It detoxifies Nitrite and Ammonia, Chlorine and Chloramine. It's the cheapest tap water conditioner I have ever found, for sure. You add a fair bit more to detoxify the bad stuff, but still so little that it costs practically nothing. A large tub will probably last you years.
I would also recommend trying some plants if your fish wont destroy them, they are good for nitrate removal. Also, with heavy fish like Oscars, get yourself a FX4 or FX6 as a treat. You won't regret it. They are overkill, but that's perfect for people like us with crap water and messy fish.
I feel your pain. My nitrates came out at 50ppm until recently, for some reason, they have gone down to 30-40ppm now. With my set up, my nitrates are gone very quickly after a water change, and I deliberately overstock to help keep aggression levels down, and I also over feed! My nitrates are almost always at 0-5ppm.
I hope this helps! It's a lot cheaper and easier than RO!