if you start with two fish, it takes longer for the ammonia to get to dangerous levels, especially in that size of tank, and the fish get almosed used to it. (that how i cyled my 20 gallon before i knew about fishless cycleing) if you absolutley need to, you can start with two really hardy fish like dainos, and with that little of a bioload it might even be like a mini cycle because the bacteria have a lot of time to catch up before the ammoina gets to high. then every week you can add 1 or 2 more fish, but you have to test your water every day (you should do that anyway) to make sure it doesnt suddenly spike to 8 or some outrageous number like that.
and about the oscar... if you want him by himself, a 75 would be ok but not perfect. you might want to consider some other cichlid that takes up less room, like severum or fire mouths, and then you can have a bit more fish