i am thinkin about getting some SD's just need some info i have a 75 gal tank and i only have one o and his bout 6in how big do the SD's have to be and what do they eat and any other info will be great thanks heaps

The sd's need to bigger than the O's mouth. If i were you i would go for 2inch ones. If you cant find or afford them you could always get some small ones and grow them in a smaller tank. SD's will quite happily eat what your oscar does and they will eat the bits that oscar spill through their gills. SD's will also eat any plants you may have growing (unlikely with Oscar)

You should be ok with 3 but you will have to do more tank maintenace