I'm buying a brand new tank 100 gallons to be precise, how many fish will it hold including an Oscar

What kind of filtration are you planning on. 100 gallons will be okay for two Oscars and probably a couple of tankmates that don't get too big. However, I would only do this if you've got extremely good filtration.

What would you recommend Oscarfishlover?

If you are referring to filtration, you can't go wrong with eheim. Arguably, they are probably the Rolls-Royce of filters. Obviously being at the top of the range comes at a price. Nobody will argue that these type of filters can be quite expensive, especially if you are dealing with large tanks. The filters are expensive, so is the media. I have never used any other type of canister filters. However, there are various others available such as fluvel. I'm sure other people will recommend other types of canister filter to you. Have a look at the review Forum, someone did a write-up on another type of filter that sounds quite good.

How much are these eheim filters if you know?

I got my two filters (eheim 2028) from Charterhouse Aquatics. I think they cost me about £130 each, including media. That was a special deal though. These are rated at 130 gallons. I have got two of them on my 125 gallon tank.

I use fluvals and i find them very good and easy maintenance. Look at the 405 with can be got for bout £90 upwards. You will need at least a 405 and a 205 but 2 405's would be better. The also do a FX5(i think) and if my memory serves me right they say it does 400g so it will do a 125 with ease